A pickup measure (formal name: anacrusis) is a partial measure of notes that precedes the downbeat (strong beat) of the first, full measure. It is sometimes also called an “incomplete measure” or an “upbeat”. The notes of the pickup measure are referred to as “pickup notes”. Think of the song “Happy Birthday”. The first strong beat in this song actually falls on the “birth” part of the word “birthday”, not on the word “happy”. Because of this, “Happy Birthday” has to start with a pickup measure. Here is what this looks like written down:
Before we start the song "Simple Man", let me explain what a "pickup measure" is.