Understanding Time Signatures

The standard model for meter in aural skills instruction is the system of time signatures or meter signs.

Time signatures are a culturally-situated system of describing cycles of beats, measures, and divisions for the purpose of notation, though they are often used informally in conversation about music that is not notated. Like other systems of conceptualizing musical time around the world, time signatures embed a series of default assumptions. These include the default assumption that meters will repeat consistently for significant spans of time, and a common association between chord changes and downbeats.

The top number of a time signature communicates how many divisions are within each beat and how many beats are contained in each measure. The table below interprets these numbers as if they were always clear and unambiguous, but that’s not always true. 2 and 4 nearly always have the meaning described here. 3 does most of the time, but occasionally it is interpreted as a single compound beat per measure. 6, 9, and 12 are nearly always interpreted as described, but occasionally indicate 6, 9, or 12 beats. In short, learn the “facts” in this table as a default, but be prepared for a messy world.

One useful way to think of this information: in simple meter, the top number tells us the number of beats in a measure; in compound meter, the top number tells us the number of beat divisions in a measure.

Other numbers, like 5, 7, and 11, are also possible on the top of a time signature. Most of the time, these numbers tell us the number of divisions in a measure. For example, 5 often means a beat of 3 divisions followed by a beat of 2 divisions or vice versa, where all the divisions are the same length and the beats vary in length. However, sometimes these numbers indicate the number of beats in the measure. This is messy; you’ll have to use clues like beaming and tempo to determine whether these numbers refer to beats (usually slower tempos) or divisions (usually medium or fast tempos).

The bottom number of a time signature tells us which note value represents a beat (simple meter) or a beat division (compound meter). This number will always be a power of 2. Because note values can occur at any speed, you can’t technically hear the difference between, say, four-four and four-eight. Nevertheless, in contemporary music-making, there is a default assumption that:

• simple meters usually have 4 on the bottom (most often two-four, three-four, or four-four), meaning the beat is represented by the quarter note, and
• that compound meters usually have 8 on the bottom (most often six-eight, nine-eight, or twelve-eight), meaning the beat division is represented by the eighth note (and the beat by the dotted quarter, since that is the length of three eighth notes).

This assumption, by the way, can sometimes help you interpret those complicated 5s, 7s, and 11s on the tops of time signatures. Most often (but not always), if 4 is on the bottom, the top number tells you the number of beats in a measure. Most often (but not always), if 8 is on the bottom, the top number tells you the number of divisions in a measure.

Still, any power of 2 is technically possible on the bottom of the time signature for any meter type. So when we present rhythmic cells, we’ll do so with different time-signature-bottom-number reference points to promote flexibility.