In music and prosody, arsis ( / ˈ ɑːr s ɪ s / ; plural arses , / ˈ ɑːr s iː z / ) and thesis ( / ˈ θ iː s ɪ s / ; plural theses , / ˈ θ iː s iː z / ) refer to the stronger and weaker parts of a musical measure or poetic foot. However, because the original definitions are unclear, writers use these terms differently. In music, arsis is an unaccented note ( upbeat ), while thesis is the downbeat. In discussions of Latin and modern poetry, arsis is often used to describe the stressed syllable of the foot, known as the ictus.

Since the terms are used in conflicting ways, Martin West, an expert on Greek meter, suggests avoiding them and using alternatives like ictus to describe the downbeat in ancient poetry. However, the use of the word ictus itself is debated.

Greek and Roman definitions

Ancient Greek writers who wrote about arsis and thesis mostly lived between the 2nd and 4th centuries AD. They likely continued a tradition that began earlier. For example, Aristides Quintilianus, who lived in the 3rd or 4th century AD, based much of his ideas on Aristoxenus, a student of Aristotle who lived in the 4th century BC. Aristoxenus wrote about rhythm.

The words arsis ("raising") and thesis ("putting down or placing") originally described the movement of a foot when marching or dancing. A Greek musicologist named Bacchius or Baccheios, who lived around the 4th century AD, explained: "Arsis means when your foot is in the air, ready to take a step. Thesis means when your foot is on the ground." Aristides Quintilianus also wrote: "Arsis is the upward motion of a body part, and thesis is the downward motion of the same part." Aristotle, who lived in the 4th century BC, said, "All walking consists of arsis and thesis."

Because rhythm and stepping were connected, parts of a rhythmic pattern were called "feet." Aristides Quintilianus wrote: "A foot is part of a rhythm that helps us recognize the whole. It has two parts: arsis and thesis."

Aristoxenus was the first writer whose surviving work used the word arsis specifically in connection with rhythm. Instead of thesis, he used the word basis ("step"). However, other Greek writers, including Plato, used the word basis to describe the whole foot, meaning the combination of arsis and thesis.

Aristoxenus often referred to arsis and thesis as "up time" and "down time," or simply "up" and "down." This idea of dividing rhythm into "up" and "down" can be traced back to Damon of Athens, a teacher of Pericles, who lived in the 5th century BC.

Stefan Hagel wrote: "Although the meaning of the ancient terms for upbeat and downbeat is debated, there is no doubt that arsis and thesis refer to a type of emphasis felt by the ancient Greeks. In instrumental music, this likely included changes in loudness, so using modern musical bars to describe rhythm makes sense."

At the same time, another definition of arsis existed. A Roman writer named Marius Victorinus, who lived in the 4th century AD, wrote: "What the Greeks call arsis and thesis—raising and putting down—describe the movement of the foot. Arsis is the lifting of the foot without sound, and thesis is the placing of the foot with sound. Arsis also means the elevation of a time, sound, or voice, and thesis means the placing-down and some kind of shortening of syllables." Lynch noted that Marius Victorinus clearly separated "lifting" (sublatio) when writing about poetic meter from "elevation" (elatio) when writing about music. Martianus Capella, who lived in the 5th century AD, made the same distinction. Lynch argued that elatio meant a rise in pitch, while others believed it meant an increase in loudness or length.

Aristides Quintilianus, when writing about rhythm rather than meter, used the second definition: "Rhythm is a combination of durations arranged in a certain order. We call their changes arsis and thesis, sound and calm."

In medical writing, the terms arsis and thesis were also used to describe the pulse of blood. Galen, who lived in the 2nd century AD, noted that this usage came from Herophilos, who lived in the 4th or 3rd century BC. He explained that the pulse itself was called arsis, and the calm after the pulse was called thesis. Galen used the same word, ēremía, for "calm" as Aristides did when talking about rhythm.

Writers who studied Greek music or dance often described the first part of a foot as the arsis or "up" part. Aristoxenus wrote: "Some feet are made of two time units, both up and down; others have three, two up and one down, or one up and two down; still others have four, two up and two down." Commentators believe Aristoxenus was referring to trochaic (– ⏑) and iambic (⏑ –) feet, saying that in trochaic feet, the long syllable is "up" (arsis), and in iambic feet, the short syllable is "up" (arsis).

Aristides Quintilianus, however, said that in some feet, thesis comes first. He wrote that an iambic foot (⏑ –) is made of an arsis and a thesis in a ratio of 1:2, while a trochaic foot (– ⏑) is made of a thesis followed by an arsis in a ratio of 2:1. Aristides referred to the sequence (– ⏑ ⏑) not as a dactyl but as "an anapaest starting from the greater" and considered it to consist of a thesis followed by a two-syllable arsis.

In the Seikilos epitaph, a piece of Greek music found on a stone inscription from the 1st or 2nd century AD, the notes in the second half of each six-time-unit bar are marked with dots called stigmai. According to a treatise known as the Anonymus Bellermanni, these dots indicate the arsis of the foot. If this is true, the thesis comes first, then the arsis.

According to Tosca Lynch, the song, when written in a 6/8 rhythm, matches the rhythm described by ancient Greek rhythmicians as an "iambic dactyl" (⏑⏔ ⁝ ⏑⏔).

In one of the fragments of music in the Anonymus Bellermanni treatise, the second two notes in a four-note bar are marked as the arsis. Stefan Hagel suggests that within the thesis and arsis, each divided into two equal parts, one note might have been stronger than the other.

In Mesomedes' Hymn to the Sun, which begins with an anapaestic rhythm (⏑⏑– ⏑⏑–), the two short syllables in each case are marked with dots, showing that the arsis comes first.

In some short examples of music in the Anonymus Bellermanni treatise, the second two notes in a four-note bar are marked as the arsis. Stefan Hagel suggests that within the thesis and arsis

Latin and English poetry

In Latin dactylic hexameter, the strong part of a foot is the first syllable, which is always long. The weak part follows it and can be either two short syllables (dactyl: long—short—short) or one long syllable (spondee: long—long). Since Classical poetry did not use stress, the arsis is usually not stressed; only the length of the syllables shows the difference.

In English, poetry uses stress, so arsis and thesis refer to the accented and unaccented parts of a foot.

Arsis and thesis in modern music

In music with a steady beat, the terms arsis and thesis describe weak and strong beats or equal parts of a measure. These terms are used in the opposite way in music compared to poetry, where arsis refers to the weak beat before the strong beat.

A fugue per arsin et thesin today often means a piece where the accents (strong beats) are shifted, making some beats weak and others strong. An example is found in a piece by Bach. In the past, this term could also describe a fugue where the theme was flipped upside down, changing its pattern.

Etymology

The Ancient Greek word ἄρσις (ársis) means "lifting, removal, or raising of the foot in beating time." It comes from the verbs αἴρω (aírō) and ἀείρω (aeírō), both meaning "I lift." In the verb αἴρω, the letter "i" is part of the present tense ending. This letter changed places with the "r" through a process called metathesis.

The Ancient Greek word θέσις (thésis) means "setting, placing, or composition." It comes from the verb τίθημι (títhēmi), which is based on the root θε/θη (the/thē). This root is repeated at the beginning of the verb in a process called reduplication. The verb τίθημι means "I put, set, or place."