Time signatures define the meter of the music. Music is "marked off" in uniform sections called bars or measures, and time signatures establish the number of beats in each. This does not necessarily indicate which beats to emphasize, however, so a time signature that conveys information about the way the piece actually sounds is thus chosen. Time signatures tend to suggest prevailing groupings of beats or pulses.
|Specific time – simple time signatures
The bottom number represents the note value of the basic pulse of the music (in this case the 4 represents the crotchet or quarter-note). The top number indicates how many of these note values appear in each measure. This example announces that each measure is the equivalent length of three crotchets (quarter-notes). You would pronounce this as "Three-Four Time" or "Three-Quarter Time".
|Specific time – compound time signatures
The bottom number represents the note value of the subdivisions of the basic pulse of the music (in this case the 8 represents the quaver or eighth-note). The top number indicates how many of these subdivisions appear in each measure. Usually each beat is composed of three subdivisions. To derive the unit of the basic pulse in compound meters, double this value and add a dot, and divide the top number by 3 to determine how many of these pulses there are each measure. This example announces that each measure is the equivalent length of two dotted crotchets (dotted quarter-notes). You would pronounce this as "Six-Eight Time".
This symbol represents 4/4 time. It derives from the broken circle that represented "imperfect" duple meter in fourteenth-century mensural time signatures.
|Alla breve or Cut time
This symbol represents 2/2 time, indicating two minim (or half-note) beats per measure. Here, a crotchet (or quarter note) would get half a beat.
Written at the start of a score, and at any significant change of tempo, this symbol precisely defines the tempo of the music by assigning absolute durations to all note values within the score. In this particular example, the performer is told that 120 crotchets, or quarter notes, fit into one minute of time. Many publishers precede the marking with letters "M.M.", referring to Maelzel's Metronome.