A Dictionary of Music and Musicians/Common Time
COMMON TIME. The rhythm of two or four beats in a bar, also called Equal time. According to the method of teaching usually observed in England, common time is divided into two kinds, Simple and Compound, Simple common time including all rhythms of two or four in a bar, except those in which the 'measure note,' or equivalent of a beat, is dotted; while a rhythm of two or four beats, each of which is dotted and therefore divisible into three, is called Compound common time. Thus 4-4 time or four crochets in a bar, and 2-4 or two crochets, are simple common times; while 6-4 or six crochets, 6-8 or six quavers, and 12-8 or twelve quavers, are compound common, because though the number of beats in a bar is even, each beat is of the value of three crochets or quavers respectively, and may be expressed by a dotted note. A better and more logical method is that taught in Germany, by which all rhythms are divided into Equal and Unequal, that is having two or three beats as a foundation, and each of these again into Simple and Compound; simple rhythms being such as have either two or three beats in a bar, the first alone accented, and compound rhythms those in which each bar is made up of two or more bars of simple time, and which have therefore two or more accents, the first being the strongest. It will be seen that according to this system, 4-4 time, which we call simple common time, will be considered as compound common, being made up of two bars of 2-4 time, just as 6-8 is compound common, being made up of two bars of 3-8 time. And this plan has the advantage that it allows for the secondary accent which properly belongs to the third beat of a bar of 4-4 time, but which is not accounted for by the theory that the time is simple.
Although the term common time is generally applied to all equal rhythms, it properly belongs only to that of four crochets in a bar, the tempo ordinario of the Italians, denoted by the sign
, which is a modernized form of the semicircle 
of the ancient 'measured music,' in which it signified the so-called 'tempus imperfectum' or division of a breve into two semibreves, in contradistinction to 'tempus perfectum' in which the breve was worth three. Another relic of the ancient time-signatures which is of importance in modern music is the sign of the 'diminutio simplex,' which was a semicircle crossed by a vertical line 
, and indicated a double rate of speed, breves being sung as semibreves, semibreves as minims, and so on. The modern form of this sign, 
, has much the same signification, and indicates the time called 'alla breve,' or two minima in a bar in quick tempo. [See Breve.]