The time signature (also known as meter signature, metre signature, and measure signature) is a convention in Western musical notation to specify how many of a particular note value are contained in each measure (bar), and in many cases how these note values are grouped into musical stresses (beats). The time signature is a notational device representing the meter, which is an auditory feature of the music. In a music score the time signature appears at the beginning as a time symbol or stacked numerals, such as or (read common time or four-four time, respectively), immediately following the key signature (or immediately following the clef symbol if the key signature is empty). A mid-score time signature, usually immediately following a barline, indicates a change of meter. The most time signatures are either simple (the note values are grouped in pairs) or compound (grouped in threes). Less-common signatures correspond to complex, mixed, additive, and irrational meters. Frequently used time signatures Simple vs. compound Simple Simple time signatures consist of two numerals, one stacked above the other: The lower numeral indicates the note value that the signature is counting. This number is always a power of 2, usually 2, 4, or 8. The upper numeral indicates how many such note values constitute a bar. If this number is 2 or 4, the signature is described as duple meter; if 3, it is called triple meter. Other, less common, descriptors such as quintuple meter are occasionally used. For instance, means two quarter-notes (crotchets) per bar, while means four eighth-notes (quavers) per bar. The most common simple time signatures are , , and . By convention, two special symbols are sometimes used for and : The symbol is sometimes used for time, also called common time or imperfect time. The symbol is typically used in place of and is called alla breve or, colloquially, cut time or cut common time. These symbols derive from Mensural time signatures, described below. Compound In compound meter, the note values specified by the bottom number are grouped into threes, and the upper number is a multiple of 3, such as 6, 9, or 12. The lower number is most commonly an 8 (an eighth-note or quaver): as in or . Beat and Subdivision Musical passages commonly feature a recurring pulse, or beat, usually in the range of 60-100 beats per minute. Depending on the tempo of the music,