In music

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...

 and prosody

Meter (poetry)

In poetry, metre is the basic rhythmic structure of a verse or lines in verse. Many traditional verse forms prescribe a specific verse metre, or a certain set of metres alternating in a particular order. The study of metres and forms of versification is known as prosody...

, arsis and thesis refer to the stronger and weaker parts of a musical measure

Bar (music)

In musical notation, a bar is a segment of time defined by a given number of beats of a given duration. Typically, a piece consists of several bars of the same length, and in modern musical notation the number of beats in each bar is specified at the beginning of the score by the top number of a...

 or poetic foot

Foot (prosody)

The foot is the basic metrical unit that generates a line of verse in most Western traditions of poetry, including English accentual-syllabic verse and the quantitative meter of classical ancient Greek and Latin poetry. The unit is composed of syllables, the number of which is limited, with a few...

. Arsis and thesis were the raising and lowering of the foot

Foot

The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...

 in beating of time, or the raising and lowering of the voice in pitch

Pitch accent

Pitch accent is a linguistic term of convenience for a variety of restricted tone systems that use variations in pitch to give prominence to a syllable or mora within a word. The placement of this tone or the way it is realized can give different meanings to otherwise similar words...

 or stress

Stress (linguistics)

In linguistics, stress is the relative emphasis that may be given to certain syllables in a word, or to certain words in a phrase or sentence. The term is also used for similar patterns of phonetic prominence inside syllables. The word accent is sometimes also used with this sense.The stress placed...

. Accordingly, in music and in Greek scansion arsis is an unaccented note (upbeat), but in Latin and modern poetry it is the stressed syllable (ictus).

Latin and English poetry

In Latin (and Greek) dactylic hexameter

Dactylic hexameter

Dactylic hexameter is a form of meter in poetry or a rhythmic scheme. It is traditionally associated with the quantitative meter of classical epic poetry in both Greek and Latin, and was consequently considered to be the Grand Style of classical poetry...

, the strong part of a foot is the first syllable — always long — and the weak part is what comes after — two short syllables (dactyl

Dactyl (poetry)

A dactyl is a foot in meter in poetry. In quantitative verse, such as Greek or Latin, a dactyl is a long syllable followed by two short syllables, as determined by syllable weight...

: long—short—short) or one long syllable (spondee

Spondee

In poetry, a spondee is a metrical foot consisting of two long syllables, as determined by syllable weight in classical meters, or two stressed syllables, as determined by stress in modern meters...

: long—long). Because Classical poetry

Classical antiquity

Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

 was not based on stress, the arsis is often not stressed; only consistent length distinguishes it.

• Arma virumque canō, Troiae quī prīmus ab ōrīs...
  Of arms and a man I sing, who first from the shores of Troy... — Aeneid
  Aeneid
  The Aeneid is a Latin epic poem, written by Virgil between 29 and 19 BC, that tells the legendary story of Aeneas, a Trojan who travelled to Italy, where he became the ancestor of the Romans. It is composed of roughly 10,000 lines in dactylic hexameter...
  
   1.1

Ar — ma vi	rum — que ca	nō — Troi	ae — quī	prī — mus ab	ō — rīs
arsis — thesis	arsis — thesis	arsis — thesis	arsis — thesis	arsis — thesis	arsis — thesis

In English, poetry is based on stress, and therefore arsis and thesis refer to the accented and unaccented parts of a foot.

Etymology

Ancient Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 ἄρσις ársis "lifting, removal, raising of foot in beating of time", from αἴρω aírō or ἀείρω aeírō "I lift". The i in aírō is a form of the present tense suffix y, which switched places with the r by metathesis

Metathesis (linguistics)

Metathesis is the re-arranging of sounds or syllables in a word, or of words in a sentence. Most commonly it refers to the switching of two or more contiguous sounds, known as adjacent metathesis or local metathesis:...

.

Ancient Greek θέσις thésis "setting, placing, composition", from τίθημι títhēmi (from root

Proto-Indo-European root

The roots of the reconstructed Proto-Indo-European language are basic parts of words that carry a lexical meaning, so-called morphemes. PIE roots always have verbal meaning like "to eat" or "to run", as opposed to nouns , adjectives , or other parts of speech. Roots never occur alone in the language...

 θε/θη, the/thē, with reduplication

Reduplication

Reduplication in linguistics is a morphological process in which the root or stem of a word is repeated exactly or with a slight change....

) "I put, set, place".