Pickup (anacrusis)
A pickup (also called an anacrusis or upbeat) is an incomplete first measure: the tune begins with one or more notes before the first bar line, on a weak beat. The first strong beat comes only after the bar line.
Many songs do not begin on beat one. A pickup may hold one beat (beat four), two beats, or even half a beat. The missing beats are usually taken from the last measure: the pickup and the last measure together make one full measure — one and three make four.
How do you start a tune with a pickup? You count in first: count the beats that the pickup leaves out, and it comes in on its own number. For a one-beat pickup in 4/4 you count “one, two, three”, and the tune begins on “four”.
A pickup is not counted as a measure of its own: measure one is the first full measure after the bar line.
What it sounds like
A pickup sounds like a breath before the phrase: a short note that leads into the strong beat. Many songs begin exactly this way, on a weak beat.
Where it is taught: the studio’s interactive textbook, chapter 13, “Ties, rests and the pickup”.