For questions about or in relation to: an anacrusis, pickup measure, or partial bar that occurs at the start of a piece (or phrase) of music. Questions on: function, notation, implementation, and performance, of anacrusis are on-topic.

An anacrusis, pickup measure, or partial bar, is an unstressed note (or group of notes), that occur at the start of a piece, section, or phrase. This allow the music to begin before the stressed downbeat of the first measure.

The incomplete bar is often compensated for – by ending the piece with another incomplete measure at the end, such that when combined these two partial bars add up to a full bar. This ensures a whole number of bars in the piece. But this is no longer considered necessary in modern practice.

Anacruses are common in poetry and melody, and are considered necessary in some style of dance.

Questions on: function, proper notation / engraving practice, software implementation, or performance practice, are all on-topic in this tag.