Answer:

Time signatures consist of two numerals, one stacked above the other:

The lower numeral indicates the note value that represents one beat (the beat unit).

The upper numeral indicates how many such beats there are grouped together in a bar.

For instance, 2/4 means two quarter-note (crotchet) beats per bar—3/8 means three eighth-note (quaver) beats per bar.

There are various types of time signatures, including: simple (such as 3/4 or 4/4), compound (e.g., 9/8 or 12/8), complex (e.g., 5/4 or 7/8), mixed (e.g., 5/8 & 3/8 or 6/8 & 3/4), additive (e.g., 3+2+3/8), fractional (e.g., 2½/4), and irrational meters (e.g., 3/10 or 5/24).

Simple Time Signatures

Basic time signatures: 4/4, also known as common time (Commontime inline.png); 2/2, also known as cut time or cut-common time (cut time); plus 2/4; 3/4; and 6/8.

Common Time Signatures

Basic time signatures: 4/4, also known as common time; 2/2, also known as cut time or cut-common time (cut time); plus 2/4; 3/4; and 6/8.

The most common simple time signatures are 2/4, 3/4, and 4/4.

Notational Variations in Simple Time

The symbol Commontime inline.png is sometimes used for 4/4 time, also called common time or imperfect time. The symbol is derived from a broken circle used in music notation from the 14th through 16th centuries, where a full circle represented what today would be written in 3/2 or 3/4 time, and was called tempus perfectum (perfect time). The symbol cut time is also a carry-over from the notational practice of late-Medieval and Renaissance music, where it signified tempus imperfectum diminutum (diminished imperfect time)—more precisely, a doubling of the speed, or proportio dupla, in duple meter. In modern notation, it is used in place of 2 and is called alla breve or, colloquially, cut time or cut common time.

Compound Time Signatures

In compound meter, subdivisions of the main beat (the upper number) split into three, not two, equal parts, so that a dotted note (half again longer than a regular note) becomes the beat unit. Compound time signatures are named as if they were simple time signatures, in which the one-third part of the beat unit is the beat, so the top number is commonly 6, 9 or 12 (multiples of 3). The lower number is most commonly an 8 (an eighth-note): as in 9/8 or 12/8.

Explanation: