bar
A segment of music contained between two bar lines, also called a measure.
In Depth
A bar (or measure) is the basic unit of musical time, defined by the time signature. In 4/4 time, each bar contains four quarter-note beats. Bar lines — vertical lines drawn through the staff — mark the boundaries between bars, helping performers keep their place in the music.
The concept of dividing music into bars emerged in the late Renaissance and became standard by the Baroque era. Before bar lines, performers had to rely entirely on the text and their own counting to stay together. The bar line was a revolutionary organisational tool that made complex ensemble music possible.
The longest bar in standard orchestral music occurs in some Bruckner symphonies, where a single bar in slow tempo can last over 15 seconds.