bar

In musical notation, a bar (or measure) is a segment of time defined by a given number of beats of a given duration. Typically, a piece consists of several bars of the same length, and in modern musical notation the number of beats in each bar is specified at the beginning of the score by the top number of a time signature (such as 3/4).

The word bar is British English, while the word measure is American English, although musicians generally understand both usages. In American English, although the words bar and measure are often used interchangeably the correct use of the word 'bar' refers only to the vertical line itself, while the word 'measure' refers to the beats contained between bars. In international usage, it is equally correct to speak of bar numbers and measure numbers, e.g. ‘bars 9–16’ or ‘mm. 9–16’. Along the same lines, it is wise to reserve the abbreviated form ‘bb. 3–4’ etc. for beats only; bars should be referred to by name in full.

The first metrically complete measure within a piece of music is called ‘bar 1’ or ‘m. 1’. When the piece begins with an upbeat (an incomplete measure at the head of a piece of music), ‘bar 1’ or ‘m. 1’ is the following measure.