In music theory, a measure (or bar) is a segment of time within a piece of music defined by a given number of beats. It is delineated by vertical lines on the musical staff. Measures are used to break down a piece of music into more manageable subdivisions. The smallest of those subdivisions are known as musical measures or musical bars. The length of a measure is determined by the time signature, which is a symbol placed at the beginning of a piece of music that indicates the number of beats in each measure and the type of note that receives one beat. For example, a time signature of 4/4 means that there are four beats in each measure, and a quarter note receives one beat.

Measures are important in music because they help musicians keep track of the rhythm and timing of a piece. They also help to organize the music into smaller, more manageable sections, making it easier to read and perform. By grouping notes into measures, composers can create a sense of structure and organization in their music, which can help to convey the emotional and expressive content of the piece.