Meter involves the way multiple pulse layers work together to organize music in time. Standard meters in Western music can be classified into simple meters and compound meters, as well as duple, triple, and quadruple meters.
Duple, triple, and quadruple classifications result from the relationship between the counting pulse (Karpinski's secondary pulse) and the pulses that are slower than the counting pulse (Karpinski's primary pulse). In other words, it is a question of grouping: how many beats (secondary pulse) occur in each bar (primary pulse). If counting-pulse beats group into twos, we have duple meter; groups of three, triple meter; groups of four, quadruple meter. Conducting patterns are determined based on these classifications.
Simple and compound classifications result from the relationship between the counting pulse and the pulses that are faster than the counting pulse. In other words, it is a question of division: does each beat divide into two equal parts, or three equal parts. Meters that divide the beat into two equal parts are simple meters; meters that divide the beat into three equal parts are compound meters.
Thus, there are six types of standard meter in Western music:
In a time signature, the top number (and the top number only!) describes the type of meter. Following are the top numbers that always correspond to each type of meter:
In simple meters, the bottom number of the time signature corresponds to the type of note corresponding to a single beat. If a simple meter is notated such that each quarter note corresponds to a beat, the bottom number of the time signature is 4. If a simple meter is notated such that each half note corresponds to a beat, the bottom number of the time signature is 2. If a simple meter is notated such that each eighth note corresponds to a beat, the bottom number of the time signature is 8. And so on.
In compound meters, the bottom number of the time signature corresponds to the type of note corresponding to a single division of the beat. If a compound meter is notated such that each dotted-quarter note corresponds to a beat, the eighth note is the division of the beat, and thus the bottom number of the time signature is 8. If a compound meter is notated such that each dotted-half note corresponds to a beat, the quarter note is the division of the beat, and thus the bottom number of the time signature is 4. Note that because the beat is divided into three in a compound meter, the beat is always three times as long as the division note, and the beat is always dotted.