Used to separate measures. Bar lines are extended to connect the upper and lower staffs of a grand staff.
In musical notation, a bar (or measure) is a segment of time defined by a given number of beats, each of which are assigned a particular note value. Dividing music into bars provides regular reference points to pin point locations within a piece of music. 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 top number of a time signature (such as 3/4), 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. 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 anacrusis (an incomplete measure at the head of a piece of music), ‘bar 1’ or ‘m. 1’ is the following measure.