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In music, alla breve (Italian for: "at the breve"; also sometimes called cut time or cut common time) refers to a musical meter notated by the time signature symbol (a C with a line through it), which is the equivalent of 2/2. Alla breve is a "simple-duple meter with a half-note pulse". Common time, notated with the time signature symbol , is the equivalent of 4/4.
In contemporary usage alla breve suggests a fairly quick tempo. Thus, it is used frequently for military marches. From about 1600 to 1900 its usage with regard to tempo varied, so it cannot always be taken to mean a quick tempo. The reason for the usage of "alla breve" is to allow the musician to read notes of short duration more cleanly with fewer beams.
Prior to 1600 the term alla breve derives from the system of mensural or proportional notation (also called proportio dupla) in which note values (and their graphical shapes) were related by the ratio 2:1. In this context it means that the tactus or metrical pulse (now commonly referred to as the "beat") is switched from its normal place on the whole note (semibreve) to the double whole note (breve).
Early music notation was developed by religious orders, which has resulted in some religious associations in notation. The most obvious is that music in triple time was called tempus perfectum, deriving its name from the Holy Trinity and represented by the "perfect" circle, which has no beginning or end.
Music in duple time was similarly called tempus imperfectum. Its symbol was the broken circle, " ", which is still used — although it has evolved to mean 4/4, or "common time", today. When cut through by a vertical line "" , it means 2/2 — "cut common", or alla breve. 
The use of the vertical line or stroke in a musical graphical symbol, as practiced in the Middle Ages and the Renaissance, and now referred to by the modern term of "cut time", did not always have the same meaning as alla breve. It sometimes had other functions, including non-mensural ones.
The following is an example with the same rhythm notated in 2/2 and in 4/4: