Bow (music)
In music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...

, a bow is moved across some part of a musical instrument
Musical instrument
A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

, causing vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

 which the instrument emits as sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

. The vast majority of bows are used with string instrument
String instrument
A string instrument is a musical instrument that produces sound by means of vibrating strings. In the Hornbostel-Sachs scheme of musical instrument classification, used in organology, they are called chordophones...

s, although some bows are used with musical saw
Musical saw
A musical saw, also called a singing saw, is the application of a hand saw as a musical instrument. The sound creates an ethereal tone, very similar to the theremin...

s and other bowed idiophone
An idiophone is any musical instrument which creates sound primarily by way of the instrument's vibrating, without the use of strings or membranes. It is the first of the four main divisions in the original Hornbostel-Sachs scheme of musical instrument classification...


Materials and manufacture

A bow consists of a specially shaped stick with other material forming a ribbon stretched between its ends, which is used to stroke the string and create sound. Different musical cultures have adopted various designs for the bow. For instance, in some bows a single cord is stretched between the ends of the stick. In the Western tradition of bowmaking—bows for the instruments of the violin
The violin is a string instrument, usually with four strings tuned in perfect fifths. It is the smallest, highest-pitched member of the violin family of string instruments, which includes the viola and cello....

 and viol
The viol is any one of a family of bowed, fretted and stringed musical instruments developed in the mid-late 15th century and used primarily in the Renaissance and Baroque periods. The family is related to and descends primarily from the Renaissance vihuela, a plucked instrument that preceded the...

 families—a hank of horsehair
Horsehair is the long, coarse hair growing on the manes and tails of horses. It is used for various purposes, including upholstery, brushes, the bows of musical instruments, a hard-wearing fabric called haircloth, and for horsehair plaster, a wallcovering material formerly used in the construction...

 is normally employed.

The manufacture of bows is considered a demanding craft, and well-made bows command high prices. Part of the bowmaker's skill is the ability to choose high quality material for the stick. Historically, Western bows have been made of pernambuco
Caesalpinia echinata is a species of Brazilian timber tree in the pea family, Fabaceae. Common names include Brazilwood, Pau-Brasil, Pau de Pernambuco and Ibirapitanga . This plant has a dense, orange-red heartwood that takes a high shine, and it is the premier wood used for making bows for...

 wood from Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

. However, pernambuco is now an endangered species whose export is regulated by international treaty, so makers are currently adopting other materials: woods such as Ipê (Tabebuia
Tabebuia is a neotropical genus of about 100 species in the tribe Tecomeae of the family Bignoniaceae. The species range from northern Mexico and southern Florida south to northern Argentina, including the Caribbean islands of Hispaniola and Cuba...

) as well as synthetic materials. These synthetic materials include carbon fiber
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

 epoxy composite and fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

. Carbon fiber bows have become very popular, and some of the better carbon fiber bows are now comparable to fine pernambuco sticks.

For the frog, which holds and adjusts the near end of the horsehair, ebony
Ebony is a dense black wood, most commonly yielded by several species in the genus Diospyros, but ebony may also refer to other heavy, black woods from unrelated species. Ebony is dense enough to sink in water. Its fine texture, and very smooth finish when polished, make it valuable as an...

 is most often used, but other materials, often decorative, are used as well; these include ivory
Ivory is a term for dentine, which constitutes the bulk of the teeth and tusks of animals, when used as a material for art or manufacturing. Ivory has been important since ancient times for making a range of items, from ivory carvings to false teeth, fans, dominoes, joint tubes, piano keys and...

 and tortoiseshell
Tortoiseshell material
Tortoiseshell or tortoise shell is a material produced mainly from the shell of the hawksbill turtle, an endangered species. It was widely used in the 1960s and 1970s in the manufacture of items such as combs, sunglasses, guitar picks and knitting needles...

. The metal parts of the frog, or mountings, may be used by the maker to mark various grades of bow, ordinary bows being mounted with nickel silver
Nickel silver
Nickel silver, also known as German silver, Argentann, new silver, nickel brass, albata,, or alpacca, is a copper alloy with nickel and often zinc. The usual formulation is 60% copper, 20% nickel and 20% zinc. Nickel silver is named for its silvery appearance, but it contains no elemental silver...

, better bows with silver
Sterling silver
Sterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925....

, and the finest being gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

-mounted. (Not all makers adhere uniformly to this practice.) Near the frog is the grip, which is made of a wire, silk, or "whalebone" wrap and a thumb cushion made of leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...

 or snakeskin
Snakeskin may refer to:*Snakeskin, a material that is made from the skin of a snake*Snakeskin , a song by Australian band Gyroscope*Snakeskin , a New Zealand film*Snakeskin , a side project of Tilo Wolff from Lacrimosa*Snakeskin Glacier...

. The tip plate of the bow may be made of bone, ivory, mammoth ivory
Elephant and mammoth ivory
Elephant and mammoth tusk ivory comes from the two modified upper incisors of extant and extinct members of the order Proboscidea. Mammoths are believed to have been extinct for 10,000 years. Because of the geographical range in Alaska and Siberia, Mammuthus primigenius tusks have been well preserved...

, or metal, such as silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...


A bow maker or Archetier
A bow maker is a person who builds, repairs, and restores ancient or modern bows for instruments of the violin family. These include violins, violas, cellos, double basses, viola d'amore, viola da gamba, etc....

 typically uses between 150 and 200 hairs from the tail of a horse for a violin bow. Bows for other members of the violin family typically have a wider ribbon, using more hairs. White hair generally produces a smoother sound and black hair (used mainly for double bass bows) is coarser, producing a rougher sound. Lower quality (inexpensive) bows often use nylon or synthetic hair. Rosin
.Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...

, a hard, sticky substance made from resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

 (sometimes mixed with wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

), is regularly applied to the bow hair to increase friction.

In making a wooden bow, the greater part of the woodworking is done on a straight stick. According to James McKean, "the bow maker graduates the stick in precise gradations so that it is evenly flexible throughout." These gradations were originally calculated by François Tourte
François Tourte
François Xavier Tourte was a Frenchman who, though trained as a watchmaker, soon changed to making bows for playing classical string instruments such as the violin....

, discussed below. In order to shape the curve or "camber" of the bow stick, the maker carefully heats the stick in an alcohol flame, a few inches at a time, bending the heated stick gradually to the proper shape. A metal or wooden template is used to get the exact model's curve and shape while heating.

The art of making wooden bows has changed little since the 19th century; most modern composite sticks roughly resemble the Tourte design. Various inventors have tried, at times, to come up with new ways of bow making; the Incredibow, for example, has a straight stick cambered only by the fixed tension of the synthetic hair.

Types of bow

Slightly different bows, varying in weight and length, are used for the violin, viola, cello, and double bass.

These are generally variations on the same basic design. However, two distinct forms of the double bass bow are in current usage. The "French" overhand bow is constructed along the same lines as the bow used with the other instruments of the orchestral string family. The French stick is grasped from opposite the frog. The "German" or "Butler" underhand bow is broader and longer than the French bow with a larger frog curved to fit the palm of the hand. The German stick is grasped with the hand encompassing the frog loosely. The German bow is the older of the two designs, having superseded the earlier arched bow. The French bow, often chosen by soloists who find it exhibits a greater range of dynamics and better control, became popular with its adoption in the 19th century by virtuoso Giovanni Bottesini
Giovanni Bottesini
Giovanni Bottesini was an Italian Romantic composer, conductor, and a double bass virtuoso.-Biography:Born in Crema, Lombardy, he was taught the rudiments of music by his father, an accomplished clarinetist and composer, at a young age and had played timpani in Crema with the Teatro Sociale before...

. Both are found in the orchestra, though typically an individual bass player prefers to perform using one or the other type of bow.


The characteristic long, sustained, and singing sound produced by the violin, viola, violoncello, and double bass is due to the drawing of the bow against their strings
Strings (music)
A string is the vibrating element that produces sound in string instruments, such as the guitar, harp, piano, and members of the violin family. Strings are lengths of a flexible material kept under tension so that they may vibrate freely, but controllably. Strings may be "plain"...

. This sustaining of musical sound with a bow is comparable to a singer using breath to sustain sounds and sing long, smooth, or legato melodies. Without the bow the violin family could only be played pizzicato
Pizzicato is a playing technique that involves plucking the strings of a string instrument. The exact technique varies somewhat depending on the type of stringed instrument....


In modern practice, the bow is almost always held in the right hand while the left is used for fingering
In music, fingering is the choice of which fingers and hand positions to use when playing certain musical instruments. Fingering typically changes throughout a piece; the challenge of choosing good fingering for a piece is to make the hand movements as comfortable as possible without changing hand...

. When the player pulls the bow across the strings (such that the frog moves away from the instrument), it is called a downbow; pushing the bow so the frog moves toward the instrument is an upbow (the directions "down" and "up" are literally descriptive for violins and violas, and are employed in analogous fashion for the cello and double bass). Two consecutive notes played in the same bow direction are referred to as a hooked bow; a downbow following a whole downbow is called a retake.

Generally, the downbow stroke is used for the strong musical beats, the upbow for weak beats. However, in the viola da gamba, it is the reverse; thus violinists, violists, and cellists look like they are "pulling" on the strong beats when they play, whereas gamba players look like they are "stabbing" on the strong beats. The difference almost certainly results from the different ways in which the bow is held in these instrument families: violin/viola/cello players hold the wood part of the bow closer to the palm, whereas gamba players use the opposite orientation, with the horsehair closer. The orientation appropriate to each instrument family permits the stronger wrist muscles (flexors) to reinforce the strong beat.

String players control their tone quality by touching the bow to the strings at varying distances from the bridge, emphasizing the higher harmonics by playing sul ponticello "on the bridge," or reducing them, and emphasizing the fundamental frequency by playing sul tasto "on the fingerboard".

Occasionally, composers ask the player to use the bow by touching the strings with the wood rather than the hair; this is known by the Italian phrase col legno
Col legno
In music for bowed string instruments, col legno, or more precisely col legno battuto , is an instruction to strike the string with the stick of the bow, rather than by drawing the hair of the bow across the strings. This results in a quiet but eerie percussive sound.Col legno is used in the final...

, "with the wood". Coll'arco, "with the bow", is the indication to use the bow hair to create the sound in the normal way.


The question of when and where the bow was invented is of interest because the bow made possible several of the most important instruments in music today. Authorities give different answers to this question, and this article will give only the predominant opinion.

Scholars are agreed that stringed instruments as a category existed long before the bow. There was a long period—possibly thousands of years—in which all stringed instruments were plucked.

In fact, it is likely that bowed instruments are not much more than a thousand years old. Eric Halfpenny, writing in the 1988 Encyclopædia Britannica, says "bowing can be traced as far back as the Islamic civilization of the 10th century ... it seems likely that the principle of bowing originated among the nomadic horse riding cultures of Central Asia, whence it spread quickly through Islam and the East, so that by 1000 it had almost simultaneously reached China, Java, North Africa, the Near East and Balkans, and Europe." Halfpenny notes that in many Eurasian languages the word for "bridge" etymologically means "horse," and that the Chinese regarded their own bowed instruments (huqin
Huqin is a family of bowed string instruments, more specifically, a spike fiddle popularly used in Chinese music. The instruments consist of a round, hexagonal, or octagonal sound box at the bottom with a neck attached that protrudes upwards...

) as having originated with the "barbarians" of Central Asia.

The Central Asian theory is endorsed by Werner Bachmann, writing in the New Grove. Bachmann notes evidence from a tenth century Central Asian wall painting for bowed instruments in what is now the city of Kurbanshaid in Tajikistan
Tajikistan , officially the Republic of Tajikistan , is a mountainous landlocked country in Central Asia. Afghanistan borders it to the south, Uzbekistan to the west, Kyrgyzstan to the north, and China to the east....


Circumstantial evidence also supports the Central Asian theory. All the elements that were necessary for the invention of the bow were probably present among the Central Asian horse riding peoples at the same time:
  • In a society of horse-mounted warriors (the horse peoples included the Huns
    The Huns were a group of nomadic people who, appearing from east of the Volga River, migrated into Europe c. AD 370 and established the vast Hunnic Empire there. Since de Guignes linked them with the Xiongnu, who had been northern neighbours of China 300 years prior to the emergence of the Huns,...

     and the Mongols), horsehair obviously would have been available.
  • Central Asian horse warriors specialized in the military bow, which could easily have served the inventor as a temporary way to hold horsehair at high tension.
  • To this day, horsehair for bows is taken from places with harsh cold climates, including Mongolia, as such hair offers a better grip on the strings.
  • Rosin
    .Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...

    , crucial for creating sound even with coarse horsehair, is used by traditional archers
    Archery is the art, practice, or skill of propelling arrows with the use of a bow, from Latin arcus. Archery has historically been used for hunting and combat; in modern times, however, its main use is that of a recreational activity...

     to maintain the integrity of the string and (mixed with beeswax
    Beeswax is a natural wax produced in the bee hive of honey bees of the genus Apis. It is mainly esters of fatty acids and various long chain alcohols...

    ) to protect the finish of the bow; for details, see these links:

From all this it is tempting to imagine the invention of the bow: some Mongol warrior, having just used rosin on his equipment, idly stroked his harp
The harp is a multi-stringed instrument which has the plane of its strings positioned perpendicularly to the soundboard. Organologically, it is in the general category of chordophones and has its own sub category . All harps have a neck, resonator and strings...

 or lyre
The lyre is a stringed musical instrument known for its use in Greek classical antiquity and later. The word comes from the Greek "λύρα" and the earliest reference to the word is the Mycenaean Greek ru-ra-ta-e, meaning "lyrists", written in Linear B syllabic script...

 with a rosin-dusted finger and produced a brief continuous sound, which caused him to have an inspiration; whereupon he seized his bow, restrung it with horsehair, and so on. Obviously, the degree to which this fantasy is true will never be known.

However the bow was invented, it soon spread very widely. The Central Asian horse peoples occupied a territory that included the Silk Road
Silk Road
The Silk Road or Silk Route refers to a historical network of interlinking trade routes across the Afro-Eurasian landmass that connected East, South, and Western Asia with the Mediterranean and European world, as well as parts of North and East Africa...

, along which goods and innovations were transported rapidly for thousands of miles (including, via India, by sea to Java). This would account for the near-simultaneous appearance of the musical bow in the many locations cited by Halfpenny.

The modern Western bow

The kind of bow in use today was brought into its modern form largely by the archetier
A bow maker is a person who builds, repairs, and restores ancient or modern bows for instruments of the violin family. These include violins, violas, cellos, double basses, viola d'amore, viola da gamba, etc....

 / bow-maker François Tourte
François Tourte
François Xavier Tourte was a Frenchman who, though trained as a watchmaker, soon changed to making bows for playing classical string instruments such as the violin....

 in 19th century France. Pernambuco wood which was imported into France to make textile dye, was found by the early French bow masters to have just the right combination of strength, resiliency, weight, and beauty. According to James McKean (reference below), Tourte's bows, "like the instruments of Stradivari
Antonio Stradivari
Antonio Stradivari was an Italian luthier and a crafter of string instruments such as violins, cellos, guitars, violas, and harps. Stradivari is generally considered the most significant artisan in this field. The Latinized form of his surname, Stradivarius, as well as the colloquial, "Strad", is...

, as still considered to be without equal."

Historical bows

The early 18th century bow referred to as the Corelli-Tartini model, is also referred to as the Italian 'sonata' bow.This basic Baroque bow supplanted by 1725 an earlier French dance bow which was quite short with a little point. The French dance bow was held with the thumb under the hair and played with short, quick strokes for rhythmic dance music. The Italian sonata bow was longer, from 24-28 inches (61–71 cm.), with a straight or slightly convex stick. The head is described as a pike's head, and the frog is either fixed (the clip-in bow) or has a screw mechanism.
The screw is an early improvement, indicative of further changes to come.
As compared to a modern Tourte-style bow, the Corelli-Tartini model is shorter and lighter, especially at the tip, the balance point is lower down on the stick, the hair more yielding, and the ribbon of hair narrower about 6 mm wide.

In the early bow (the Baroque bow), the natural bow stroke is a non-legato norm, producing what Leopold Mozart called a "small softness" at the beginning and end of each stroke.

A lighter, clearer sound is produced, and quick notes are cleanly articulated without the hair leaving the string.

A truly great example of such a bow, described by David Boyden, is part of the Ansley Salz Collection at the University of California at Berkeley.
It was made around 1700, and is attributed to Stradivari.

Towards the middle of the century (18th century), there was a move into the Transitional period, the separation of hair from stick became greater, particularly at the head. This greater separation is necessary because the stick becomes longer and straighter, approaching a concave shape.

Up until the advent of the bow by Tourte, there was absolutely no standardization of bow features during this Transitional period, and every bow was different in weight, length and balance.
In particular, the heads varied enormously by any given maker.

Another transitional type of bow may be called the Cramer bow, after the violinist Wilhelm Cramer
Wilhelm Cramer
Wilhelm Cramer was a famous London violinist and musical conductor of German origin. He was one of a numerous family who were identified with the progress of music during the 18th and 19th centuries...

 (1746–99) who lived the early part of his life in Mannheim (Germany) and, after 1772, in London. This bow and models comparable to it in Paris, generally prevailed between the gradual demise of the Corelli-Tartini model and the birth of the Tourte-that is, roughly 1750-85.
In the view of top experts, the Cramer bow represents a decisive step towards the modern bow.

The Cramer bow and others like it were gradually rendered obsolete by the advent of François Tourte
François Tourte
François Xavier Tourte was a Frenchman who, though trained as a watchmaker, soon changed to making bows for playing classical string instruments such as the violin....

's standardized bow. The hair (on the Cramer bow) is wider than the Corelli model but still narrower than a Tourte, the screw mechanism becomes standard, and more sticks are made from pernambuco, rather than the earlier snakewood, ironwood, and china wood, which were often fluted for a portion of the length of the stick.

Fine makers of these Transitional models were Duchaîne, La Fleur, Meauchand, Tourte père, and Edward Dodd
Edward Dodd
Edward Dodd was a U.S. Representative from New York.Born in Salem, New York, Dodd attended the public schools.He engaged in mercantile pursuits.He moved to Argyle, New York, in 1835....


The underlying reasons for the change from the old Corelli-Tartini model to the Cramer and, finally, to the Tourte were naturally related to musical demands on the part of composers and violinists.
Undoubtedly the emphasis on cantabile, especially the long drawn out and evenly sustained phrase, required a generally longer bow and also a somewhat wider ribbon of hair. - David Boyden These new bows were ideal to fill the new, very large concert halls with sound and worked great with the late classical and the new romantic repertoire.

Today, with the rise of the historically informed performance
Historically informed performance
Historically informed performance is an approach in the performance of music and theater. Within this approach, the performance adheres to state-of-the-art knowledge of the aesthetic criteria of the period in which the music or theatre work was conceived...

 movement, string players have developed a revived interest in the lighter, pre-Tourte bow, as more suitable for playing stringed instruments made in pre-19th century style.

Other types of bow

The Chinese yazheng
The yazheng is a Chinese string instrument. It is a long zither similar to the guzheng but bowed by scraping with a sorghum stem dusted with resin, a bamboo stick, or a piece of forsythia wood...

 and yaqin, and Korean ajaeng
The ajaeng is a Korean string instrument. It is a wide zither with strings made of twisted silk, played by means of a slender stick made of forsythia wood, which is scraped against the strings in the manner of a bow...

 zithers are generally played by "bowing" with a rosined stick, which creates friction against the strings without any horsehair. The hurdy-gurdy's strings are similarly set into vibration by means of a "rosin wheel," a wooden wheel which contacts the strings as it is rotated by means of a crank handle, creating a "bowed" tone.


Careful owners always loosen the hair on a bow before putting it away. James McKean (reference below) recommends that the owner "loosen the hair completely, then bring it back just a single turn of the button." The goal is to "keep the hair even but allow the bow to relax."

Since hairs may break in service, bows must be periodically rehaired, an operation usually performed by professionals rather than by the instrument owner.

Bows sometimes lose their correct camber (see above), and are recambered using the same heating method as is used in the original manufacture.

Lastly, the grip or winding of the bow must occasionally be replaced to maintain a good grip and protect the wood.

These repairs are best left to professionals, as the head of the bow is extremely fragile, and a poor rehair, or a broken ivory plate on the tip can lead to ruining the bow.


In vernacular speech the bow is occasionally called a fiddlestick. Bows for particular instruments are often designated as such: "violin bow", "cello bow", and so on.

See also

  • Pernambuco
    Caesalpinia echinata is a species of Brazilian timber tree in the pea family, Fabaceae. Common names include Brazilwood, Pau-Brasil, Pau de Pernambuco and Ibirapitanga . This plant has a dense, orange-red heartwood that takes a high shine, and it is the premier wood used for making bows for...

  • Playing the violin, section on "Bowing techniques"
  • Rosin
    .Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...

  • String instrument, section on "Bowing"
  • Musical bow
    Musical bow
    The musical bow is a simple string musical instrument most archaic cultures as well as in many in the present day. It consisting of a string supported by a flexible stick 1.5 to 10 feet long, and strung end to end with a taut cord. Usually made out of wood...

    , musical instrument
  • Bowed guitar
    Bowed guitar
    Bowed guitar is a method of playing a guitar, acoustic or electric, in which the guitarist uses a bow to play the instrument, similar to playing a cello or a viola da gamba. Unlike other bowed instruments, the guitar has a flat bridge, making it difficult to bow individual notes on the middle strings...

  • Curved bow
    Curved bow
    The curved bow for string instruments enables string players to control the tension of the bow hairs in order to play one, two, three and four strings simultaneously and to change easily among these possibilities...

Further reading

External links

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