A bar, also referred to as measure, is a small, uniform, unit of time duration in music that makes it much easier to read and follow written music and also facilitates its simultaneous performance by multiple musicians.
Each bar in a work, or in a section thereof, contains an equal number of beats. The number of beats is indicated by the top number in the time signature, which is located at the beginning of the work or section.
Adjacent bars in standard music notation are usually separated by a single, thin vertical line called a bar line. Bars can be sequentially numbered in order to provide clear reference points for identifying every location within a score, further adding to the ease of reading and simultaneous performance by multiple musicians.