Anacrusis

Sing "Happy Birthday" tapping your foot on the strong beat. Your foot does not come down on "Hap", nor on "py". It comes down on "birth". The first two syllables arrive before the first strong beat, and the song has in fact begun before its first measure:

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Those notes arriving before the first strong beat are the anacrusis, also called the pickup or upbeat.

Three ways to begin

Music does not always begin on the first beat; it can begin on any part of the measure (Lacerda 1966, ch. IX). Med names the three possibilities (Med 1996, ch. XXII).

Thetic rhythm. It begins on the first beat of the measure, the strong beat. The name comes from the Greek thesis, the beat below, the foot planted on the ground in dance:

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Anacrustic rhythm. The opening notes come before the measure begins, as in "Happy Birthday":

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Acephalous rhythm, the "headless" one. The start of the first measure is taken by a rest, and the music comes in after it, on a weak beat:

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The word anacrusis also comes from Greek: anakrousis named the syllables that, at the start of certain lines of verse, came before the stressed syllable (Med 1996, ch. XXII). That is exactly what the music does.

Anacrusis or acephalous: the half rule

Anacrustic and acephalous openings look alike, and the difference lies in how they are written. Lacerda gives the rule (Lacerda 1966, ch. IX):

• when the opening notes fill more than half of a duple or quadruple measure, or more than two thirds of a triple one, the whole measure is written, starting with rests. That is the acephalous rhythm;
• when they fill less than that, only the notes are written, without completing the measure. That is the anacrusis.

Med gives the same division, with the same limits (Med 1996, ch. XXII). In the acephalous example above, the notes fill three of the four beats, more than half, and the measure is written in full, with the rest. In the anacrustic one, the note fills a single beat and stands alone before the first barline.

In practice the anacrusis tends to be short. Blatter notes that it usually has only one or two beats, standing for the last beat, or the last two, of a measure (Blatter 2007, part 1, art. 4).

One consequence of the rule: no rests are written before an anacrusis (Med 1996, ch. XXII). If the music begins on the last beat, the score begins on the last beat. Filling the start with rests would turn the anacrusis into an acephalous measure it is not.

The last measure settles the account

If the first measure is incomplete, the custom is to leave the last one incomplete too, so that the two together make a whole measure (Lacerda 1966, ch. IX). In "Happy Birthday" the anacrusis has one beat and the last measure two: together, the three beats of a triple measure.

Blatter confirms the custom and adds that there are many exceptions (Blatter 2007, part 1, art. 4). Med says the same with an "or not": the anacrusis is balanced, or not, in the last measure (Med 1996, ch. XXII). The reason for the custom is musical: in a song with several verses, the end of one runs into the anacrusis of the next, and the count of measures stays right.

When measures are numbered, the anacrusis does not count as measure 1. It belongs to measure "number zero" (Med 1996, ch. XXII), and the first complete measure is number 1.

Words ask for an anacrusis

Med points out that the accents of words should coincide with metric accents (Med 1996, ch. XXII). That is what makes the anacrusis so common in songs.

Many English phrases begin with unstressed syllables: "the night", "around", "before". For the strong syllable to land on the strong beat, the weak ones before it have to land before that beat, and the phrase begins with an anacrusis. "Hap-py birth-day" is the same case: two syllables leading into the one that carries the downbeat.

The idea also works inside the music. In the broader sense, an anacrusis is any group of notes leading into a point of arrival: a change of key, of rhythm, of tempo (Med 1996, ch. XXII). Many phrases inside a piece begin with an anacrusis of their own, even when the piece itself began on the strong beat.

Masculine and feminine endings

The beginning has three forms; the ending has two (Med 1996, ch. XXII).

A masculine ending closes on the strong beat of the measure:

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A feminine ending closes on a weak beat:

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The masculine ending sounds firm and conclusive; the feminine one gentler, like a line ending on an unstressed syllable. And since any beginning can pair with any ending, Med counts six combinations: thetic, anacrustic or acephalous, each with a masculine or feminine ending (Med 1996, ch. XXII).

What comes next

The anacrusis shows that meter does not start on paper but in the ear: the first strong beat is where the music leans, wherever the first note happens to be. The next step in organising time is irregular divisions, tuplets, which fit three notes where two would go.