A measure, also referred to as bar, is a small, uniform, unit of duration in music. The use of measures makes it much easier to read and follow written music and also facilitates its simultaneous performance by multiple musicians.
Each measure 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 measures are usually separated by a single, thin vertical line called a bar line. Measures 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.