A measure is a segment of musical time defined by a given number of beats. Vertical bar lines on the staff mark the boundaries of each measure. The time signature tells how many beats the measure contains and what note value receives one beat.

Grouping beats into measures gives music a regular hierarchical structure. Strong and weak beats within the measure create the sense of meter. Composers and performers use the measure as a convenient unit for organizing phrases, counting rests, and coordinating ensemble playing.

In some music the measure length remains constant; in others it changes. Bar lines are a notational convenience rather than an absolute acoustic fact, yet they strongly influence how musicians feel and count the music. Learning to read and internalize measures is one of the first steps in musical literacy.

• Segment of time containing a set number of beats
• Marked by bar lines
• Defined by the time signature
• Organizes meter and phrasing