The time signature appears at the beginning of a piece of music, right after the key signature. Unlike the key signature, which is on every staff, the time signature will not appear again in the music unless the meter changes. The meter of a piece of music is its basic rhythm; the time signature is the symbol that tells you the meter of the piece and how (with what type of note) it is written.
The time signature appears at the beginning of the piece of music, right after the clef symbol and key signature.
Beats and Measures
Because music is heard over a period of time, one of the main ways music is organized is by dividing that time up into short periods called beats. In most music, things tend to happen right at the beginning of each beat. This makes the beat easy to hear and feel. When you clap your hands, tap your toes, or dance, you are "moving to the beat". Your claps are sounding at the beginning of the beat, too. This is also called being "on the downbeat", because it is the time when the conductor's baton hits the bottom of its path and starts moving up again.
The downbeat is the strongest part of the beat, but some downbeats are stronger than others. Usually a pattern can be heard in the beats: strong-weak-weak-strong-weak-weak, or strong-weak-strong-weak. So beats are organized even further by grouping them into bars, or measures. (The two words mean the same thing.) For example, for music with a beat pattern of strong-weak-weak-strong-weak-weak, or 1-2-3-1-2-3, a measure would have three beats in it. The time signature tells you two things: how many beats there are in each measure, and what type of note gets a beat.
Reading the Time Signature
This time signature means that there are three quarter notes (or any combination of notes that equals three quarter notes) in every measure. A piece with this time signature would be "in three four time" or just "in three four".
Meter: Reading Time Signatures
Most time signatures contain two numbers. The top number tells you how many beats there are in a measure. The bottom number tells you what kind of note gets a beat.
In "four four" time, there are four beats in a measure and a quarter note gets a beat. Any combination of notes that equals four quarters can be used to fill up a measure.
You may have noticed that the time signature looks a little like a fraction in arithmetic. Filling up measures feels a little like finding equivalent fractions, too. In "four four time", for example, there are four beats in a measure and a quarter note gets one beat. So four quarter notes would fill up one measure. But so would any other combination of notes that equals four quarters: one whole, two halves, one half plus two quarters, and so on.
A few time signatures don't have to be written as numbers. Four four time is used so much that it is often called common time, written as a bold "C". When both fours are "cut" in half to twos, you have cut time, written as a "C" cut by a vertical slash.
Counting and Conducting
You may have already noticed that a measure in four four time looks the same as a measure in two two. After all, in arithmetic, four quarters adds up to the same thing as two halves. For that matter, why not call the time signature "one one" or "eight eight"?
Measures in all of these meters look the same, but feel different. The difference is how many downbeats there are in a measure.
Or why not write two two as two four, giving quarter notes the beat instead of half notes? The music would look very different, but it would sound the same, as long as you made the beats the same speed.
The music in each of these staves should sound exactly alike.
So why is one time signature chosen rather than another? The composer will normally choose a time signature that makes the music easy to read and also easy to count and conduct. Does the music feel like it has four beats in every measure, or does it go by so quickly that you only have time to tap your foot twice in a measure?
A common exception to this is six eight time, and the other time signatures (for example nine eight and twelve eight) commonly used to write compound meters. A piece in six eight might have six beats in every measure, with an eighth note getting a beat. But it is more likely that the conductor will give only two beats per measure, with a dotted quarter (or three eighth notes) getting one beat. Since beats normally get divided into halves and quarters, this is the easiest way for composers to write beats that are divided into thirds. In the same way, three eight may only have one beat per measure; nine eight, three beats per measure; and twelve eight, four beats per measure.
In six eight time, a dotted quarter usually gets one beat. This is the easiest way to write beats that are evenly divided into three rather than two.
Schmidt-Jones, C.Time Signature, Connexions Web site. http://cnx.org/content/m10956/2.11/, Jan 5, 2010.