Two numbers written at the beginning of a piece of music that indicate the number of beats per measure and which type of note gets one beat. 4/4 time, in which there are 4 beats per measure and a quarter note receives one beat, is the most common time signature.

Waltzes are in 3/4 time, and Dave Brubeck's Take Five is in 5/4 time. Classical and Jazz are two of the few music genres which use time signatures other than 4/4.

Time Signature indicates how to count a piece of music. This is extremely important, for music is timed sound and silence. And the musician is his own clock

The top number tells how many are counted; the bottom number tells of what.

Time Signature also indicates the emphasis of the piece, which can only be figured out by counting.

The first beat in the bar--the down beat--is always the most emphatic, loudest, or accented. In 2/4, the second beat--the up beat--is, of course, the lessor accented, or weak beat.

In 3/4 time, waltz time, the down beat is accented, and beats 2 abd 3 are weak beats.

In 4/4 time, the pattern is strong beat, weak beat, medium weak beat, weak beat.

In 6/8 time--6 beats of an eighth note each--the pattern is strong weak weak, medium weak weak. Alternatively, one could conceive 6/8 time as being 2 beats composed of 3 eighth notes each, instead of the usual 2 eighth notes to each beat.

This is why time signatures with 8 on the bottom are called compound and not simple as time signatures with 4 on the bottom are.

For more on counting, see metronome.

Just a few more facts on time signatures:

In ska, reggae, and a few other genres, the 2nd and 4th beats (up beats) of the 4/4 signature are accented.

In Scandinavian waltzes, the 2nd beat is accented. Think of standard waltz ad OOM pah pah. Scandinavian waltzes go oom PAH oom. :)

Think of 6/8 and 9/8 time signatures as triplet versions of 2/4 and 3/4. 6/8 has two pulses per measure, and 9/8 has three. The difference between 2/4, 3/4 and 6/8, 9/8 is that in 6/8 and 9/8, each pulse contains three notes.

In rock, a 6/8 beat may be used to sound fast paced because 16th notes in 4/4 are too fast, 8th notes are too slow, but the triplet (12th) note is perfect (that's my theory, anyway).

Various non-conventional time signatures can lead to confusing (at times) but interesting rhythmic feels. I wrote a song in 10/8 once, where parts of it sounded like 5/4, but other parts of the exact same piece sounded like 4/4 but repeated 5 times instead of 4 (as in standard 4/4 music) . Sometimes, time signatures are alternated every other measure.
Gustav Holst's Song Of the Blacksmith alternates between 3/4 and 4/4.
Toadies by Possum Kingdom uses an unusual time signature which is mainly 4/4 but counts like this, "1 2 3 4 1 2 3 4 1 2 3 4 1 2 1 2 3 4 1 2 3 4 1 2 3 4" (repeats).

Sometimes unusual time signatures are good because just as you are listening, your brain naturally starts counting 1 2 3 4 1 2 3 4 and if all of a sudden you're thrown off, and the music becomes instantly more interesting.

A time signature in music notation is drawn on the staff immediately after the clef. But while the clef is drawn on every staff, the time signature is only drawn on the first staff or grand staff, until the composer changes it during the course of the piece.

Time signature is used to indicate how the beats in the music are divided between measures. It consists of two numbers, one atop the other. The top number indicates how many beats there are per measure. The bottom number indicates what type of note receives a single beat -- 1 means a whole note, 2 a half note, 4 a quarter note, 8 an eighth note, 16 a sixteenth note. For instance, 3/4 would mean three quarter notes per measure (a nice waltz), 2/2 would mean two half notes per measure, and 7/8 would mean seven eighth notes per measure.

A time signature on the grand staff looks something like this, although in proper sheet music the top number spans the top two spaces in the staff and the bottom number spans the bottom two spaces:

         /\
  /|----| /-------
 | |    |/      
 | |----/---------
 | |   /|       3
 | |--/-|/\-------
 | | |  |  |    4
 | |-| -|--|------
 | |  \ |  |    
 | |---\|_/-------
 | |    |
/  |   \|
\  |
 | |
 | |--------------
 | |  /   \  * 
 | |-*-----|------
 | |       | *  3
 | |------/-------
 | |     /      4 
 | |----/ --------
 | |   /       
  \|--------------

Two time signatures are so common they have special symbols. These are called common time (4/4 time) and cut time (2/2 time).

I have a few more points to add on the subject of time signatures.

My first point may seem a little picky, but it is true that a time signature is not a fraction. It is notated in common language quite frequently as a fraction, but in musical notation, you will notice, there is no line between the two numbers (see mblase's wu directly above). I would prefer if time signatures, in common language, would be notated with a dash instead of a slash (4-4 instead of 4/4), but most people would know what I meant by 4/4, more than they would by 4-4. And anyway, this is a pretty minor point.

Something that is a little more important to keep in consideration (especially in regards to the accented pulses noted by themusic and chrisjh), is that these "rules" are nothing more than conventions. There is nothing stopping a musician from playing 6/8 as if it were a simple time signature, or 3/4 as if it were compound. The same is true about the accents in the messure. These are the conventions followed by most composers and musicians of the "classical" fold, and can be used as the basis for musical knowledge. When expanded upon, these conventions loose all hints of meaning.

As a student of music, I am well aware that the above paragraph will be looked down upon by many musicians. I have gotten into many a fight with many a teacher on this very subject. As two examples of 6/8 being played as a simple signature (thus proving that any signature can be played contrary to the popular convention), I point to two songs, one easier to find than the other. David Byrne's Crash (off of his DavidenryB album) has an outro section that does this. On Live and In Living Colour, a live album recorded by Living Colour after the release of Vivid, as it was broadcast over WNEW from New York City, has an extended drum solo just before Money Talks. Will Calhoun does play 6/8 as a simple time signature during one part of that solo (for those of you who have this amazing performance, it's the part where he's playing along with the synth loop).

Time signature. (Music)

A sign at the beginning of a composition or movement, placed after the key signature, to indicate its time or meter. Also called rhythmical signature. It is in the form of a fraction, of which the denominator indicates the kind of note taken as time unit for the beat, and the numerator, the number of these to the measure.

 

© Webster 1913