The viola
is a bowed

Bow (music)

In music, a bow is moved across some part of a musical instrument, causing vibration which the instrument emits as sound. The vast majority of bows are used with string instruments, although some bows are used with musical saws and other bowed idiophones....

 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...

. It is the middle voice of the violin family

Violin family

The violin family of musical instruments was developed in Italy in the sixteenth century. The standard modern violin family consists of the violin, viola, cello, and double bass....

, between the violin

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 the cello

Cello

The cello is a bowed string instrument with four strings tuned in perfect fifths. It is a member of the violin family of musical instruments, which also includes the violin, viola, and double bass. Old forms of the instrument in the Baroque era are baryton and viol .A person who plays a cello is...

.
The viola is similar in material and construction to the violin

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....

. A full-size viola's body is between 1 inches (2.5 cm) and 4 inches (10.2 cm) longer than the body of a full-size violin (i.e., between 15 and 18 in (38.1 and 45.7 cm)), with an average length of 16 inches (40.6 cm). Small violas for children typically start at 12 inches (30.5 cm), which is equivalent to a half-size violin.