U+1D102 Musical Symbol Final Barline

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In musical notation, a bar (or measure) is a segment of time corresponding to a specific number of beats in which each beat is represented by a particular note value and the boundaries of the bar are indicated by vertical bar lines. Dividing music into bars provides regular reference points to pinpoint locations within a musical composition. It also makes written music easier to follow, since each bar of staff symbols can be read and played as a batch.

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 time signature. In simple time, (such as ³
₄), the top figure indicates the number of beats per bar, while the bottom number indicates the note value of the beat (the beat has a quarter note value in the ³
₄ example).

The word bar is more common in British English, and the word measure is more common in American English, although musicians generally understand both usages. The twelve-bar blues, however, is always "twelve-bar blues". 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 usually recommended 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 bar within a piece of music is called "bar 1" or "m. 1". When the piece begins with an anacrusis (an incomplete bar at the head of a piece of music), "bar 1" or "m. 1" is the following bar.