Time signature
The fixed counting unit of a bar.
What it means
The time signature fixes how many beats a bar has and which note value forms the beat. The fraction 3/4 means: three beats, each a quarter note. The upper number names the quantity, the lower the note value.
Simple and compound time signatures are distinguished. In the simple ones — 2/4, 3/4, 4/4 — the beat divides into two parts. In the compound ones — 6/8, 9/8, 12/8 — it divides into three; there the fraction names not the beats but their thirds, and the actual beat is the dotted quarter. Alongside these stand the irregular time signatures such as 5/4 or 7/8, whose groups are of unequal size.
To be kept apart from the time signature is the tempo: it says nothing about how fast to play, only how time is organized.
In the notation
The time signature stands at the beginning of the line after the clef and key signature, not on every line, but only at the beginning and at a change. Instead of digits, the signs C for common time and ¢ for cut time occasionally stand. The beaming of eighths and sixteenths follows the time signature and makes its organization visible in the notation.
Musical use
The choice of time signature shapes the character of a piece and is closely connected with the genre: waltz and minuet in three-four time, march and polka in two-four time, gigue and tarantella in six-eight time. In twentieth-century music the time signature is handled more flexibly, up to bar-by-bar changes. Where music runs in large arcs without a fixed metre, the time signature is occasionally omitted altogether.
Performance practice
Look before playing at which unit is actually the beat — with compound time signatures it is not the note value in the denominator. Check what a metronome marking refers to. Feel the accents, instead of treating all beats the same. And with a metre change, watch which note value stays the same: this decides whether the tempo changes.