The time signature is written as a fraction at the start of a work: the top number indicates how many beats are in a bar, the bottom the duration by which one beat is expressed. Thus 3/4 means three quarter-note beats to the bar (the waltz metre), and 4/4 four (the most common “straight” metre).
Time signatures may be simple (2/4, 3/4), compound (6/8, 9/8, 12/8), or mixed/asymmetric (5/8, 7/8), characteristic of Balkan and Caucasian folk music and of modern music. The time signature sets the metric “grid” onto which the rhythm falls, and at the same time the character: march-like, dance-like, flowing. A change of time signature within a work creates an effect of surprise. The time signature is one of the first things a musician reads from a score — the key to a correct feel for the music's pulse.
