Bar (music) - Bar

Originally, the word bar derives from the vertical lines drawn through the staff to mark off metrical units and not the bar-like (i.e., rectangular) dimensions of a typical measure of music. In British English, these vertical lines are called bar, too, but often the term bar-line is used in order to make the distinction clear. In American English, the word bar stands for the lines and nothing else. A double bar-line (or double bar) can consist of two single bar-lines drawn close together, separating two sections within a piece, or a bar-line followed by a thicker bar-line, indicating the end of a piece or movement. Note that the term double bar refers not to a type of bar (i.e., measure), but to a type of bar-line. Another term for the bar-line denoting the end of a piece of music is music end.

A repeat sign (or, repeat bar-line) looks like the music end, but it has two dots, one above the other, indicating that the section of music that is before is to be repeated. The beginning of the repeated passage can be marked by a begin-repeat sign; if this is absent the repeat is understood to be from the beginning of the piece or movement. This begin-repeat sign, if appearing at the beginning of a staff, does not act as a bar-line because no bar is before it; its only function is to indicate the beginning of the passage to be repeated.

In music with a regular meter, bars function to indicate a periodic accent in the music, regardless of its duration. In music employing mixed meters, bar-lines are instead used to indicate the beginning of rhythmic note groups, but this is subject to wide variation: some composers use dashed bar-lines, others (including Hugo Distler) have placed bar-lines at different places in the different parts to indicate varied groupings from part to part.

One of the equal parts into which a piece of music is divided containing a certain number of beats.

The bar line is much, much more than a mere accent, and I don't believe that it can be simulated by an accent, at least not in my music. —Igor Stravinsky,

Bars and bar-lines also indicate grouping: rhythmically of beats within and between bars, within and between phrases, and on higher levels such as meter.