Not all pieces of music begin on the first beat of a bar. Any notes written before the first full bar are called lead-in notes (or pick-up notes). When lead-in notes occur, the final bar of the piece is also incomplete. The lead-in notes and the notes in the last bar add up to one full bar. Here are some examples.
This song is an early Jazz standard made popular by brass bands in New Orleans. It contains three lead-in notes. On the recording there are five drumbeats to introduce this song.
This symbol is the common time signature.
Common time is the same as * time.
In this duet there is only one lead-in note.