Bar - Line

Bar
In written Western music the bar-line came to be used, a vertical line through the stave, to mark metrical units or bars (= measures). By the later 17th century the bar-line had come to be used immediately preceding a strong beat, so that a bar came to begin normally with an accented note. The double bar or double bar-line marks the end of a section or piece

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Bar

In musical notation, a bar (or measure) is a segment of time defined as 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 a time signature (such as 3/4).
The word measure is heard more frequently in the U.S., while bar is used in other English-speaking countries, although musicians generally understand both usages. The word bar derives from the vertical lines which separate one measure from another, and not the bar-like (i.e., rectangular) dimensions of a typical measure of music.
Although the words bar and measure are often used interchangably the correct use of the word 'bar' refers only to the vertical line itself, while the word 'measure' refers to the music contained between bars.[1] It is thus more correct to think of measure numbers rather than bar numbers, and many critical editions refer to specific locations in this way e.g. m.22, mm.9-16 etc. Actual critical editions using this nomenclature should always include a key to distinguish between bars and beats and it is more precise to reserve the abbreviated form b.1 / bb.3-4 etc. for beats only; bars should be referred to by name in full.

The bar number is itself derived from the measure number and so the measure always follows the bar and while m.1 marks the first metrically complete measure within a piece of music, bar 1 is often left off the score. The exception to this is when the piece begins with a cue measure (again, often confused with the term the "cue bar"); in this instance, a cue measure is the first incomplete measure at the head of a piece of music or of a complete movement within a larger composition. By definition, the cue measure contains only a portion of the total number of beats required to make up a complete measure and is properly understood as a zero measure or m.0 followed by bar 1.

First and second time measures should share the same number but are easily distinguished by the suffix, e.g. m.16a (first time measure) / m.16b (second time measure) etc.

Provided by http://en.wikipedia.org/wiki/Bar_(music)