In musical notation, a bar (or measure) is a segment of music bounded by vertical lines, known as bar lines (or barlines), usually indicating one or more recurring beats. The length of the bar, measured by the number of note values it contains, is normally indicated by the time signature.
A time signature (also known as meter signature, metre signature, and measure signature) is an indication in music notation that specifies how many note values of a particular type fit into each measure (bar). The time signature indicates the meter of a musical movement at the bar level.
In a music score the time signature appears as two stacked numerals, such as
4 (spoken as four–four time), or a time symbol, such as 
(spoken as common time). It immediately follows the key signature (or if there is no key signature, the clef symbol). A mid-score time signature, usually immediately following a barline, indicates a change of meter.