In musical notation, a bar or measure is a segment of time defined as a given number of beats of a given duration. 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.
A bar line (or barline) is a vertical line which separates bars. A double bar can consist of two single barlines drawn close together, separating two sections within a piece, or a barline followed by a thicker barline, indicating the end of a piece or movement. A repeat barline looks like the second type of a double bar but has two more 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 barline; if this is absent the repeat is understood to be from the beginning of the piece or movement. This begin-repeat barline, if appearing at the beginning of a staff, does not act as a true barline because no bar is before it; its only function is to indicate the beginning of the passage to be repeated.
Note that the term double bar refers not to a type of bar (i.e., measure), but to a type of barline. Music end is a term for the barline denoting the end of a piece of music.
In music with a regular meter, bars function to indicate a periodic agogic accent in the music. Traditionally the first beat of each bar is slightly accented, regardless of its duration. In music employing mixed meters, barlines are instead used to indicate the beginning of rhythmic note groups, but this is subject to wide variation: some composers use dashed barlines, others (including Hugo Distler) have placed barlines at different places in the different parts to indicate varied groupings from part to part.
A hypermeasure, large-scale or high-level measure, or measure-group is a metric unit in which, generally, each regular measure is one beat (actually hyperbeat) of a larger meter. Thus a beat is to a measure as a measure/hyperbeat is to a hypermeasure. Hypermeasures must be larger than a notated bar, perceived as a unit, consist of a pattern of strong and weak beats, and along with adjacent hypermeasures, which must be of the same length, create a sense of hypermeter. The term was coined by Edward T. Cone.
