A time signature lets musicians know how many beats are in a measure.
One of the first things you will see when looking at a piece of music is the time signature. A time signature tells you two important things:
Time signatures will look like two numbers on top of each other, similar to a fraction.
The top number tells you how many beats are in the measure. The bottom number tells you what note value will get the beat. You might be wondering, how do I know what note value is a 4? Try answering this: It would take 4 _______ to equal a whole note. If you said "quarter notes" you are right! So, seeing a 4 means the quarter note will get the beat. You can always determine which note will get the beat by figuring out the note value would take that many of to equal a whole note. Looking at the example above, we know that the quarter note will get the beat (4) and there will be three (3) beats per measure. Here is an example of music in 3/4 time. You can see the count of the beats under the measures.
You know how to figure out which note value gets the beat, but to make things easier, here is a time signature cheat sheet. If the bottom number is a:
/1 - The whole note gets the beat
/2 - Half notes get the beat
/4 - Quarter notes get the beat
/8 - Eighth notes get the beat
/16 - Sixteenth notes get the beat
See if you can say which note receives the beat and how many beats are in a measure for these examples:
The quarter note (4) gets the beat and there are 4 beats per measure. This is the most common time signature most beginning musicians see, It's also what most popular music is written in. Since it is so common, it's nicknamed "common time," and can be written as the letter "C" instead of "4/4."
In 2/2 time, the half note gets the beat (2) and there are 2 beats per measure. This is sometines known as "cut time," since it is exactly half of common time (or 4/4). You will sometimes see this written as a C with a line through it (like the cent symbol in American currency).
In 6/8 time, the eighth note gets the beat (8) and there are 6 beats per measure. This common time signature can be "in 6" or "in 2." When it is counted in six, it means you could every eight note (1-2-3-4-5-6). When it is "in 2," you will only be counting beats 1 and 4. (1-2-3-4-5-6). When the music is a quick tempo, you will usually default to the "in 2," style of counting 6/8.
In music, the word meter is used to describe the pattern of beats. The time signature tells the musician the pattern of the beats, so "time signature" and "meter" are related. Time signatures tell you what the meter is. To understand the differences between simple and compound meter, you should first understand how notes can be divided into two parts or (if they are dotted rhythms) divided into three parts.
The term simple meter is used to describe meters where the beat can be divided into two equal parts. This would be meters like 2/4, 4/4, and 3/8.
You can have simple meter where there are two groups to the measure (like in 2/4, where there are three groups to the measure (like 3/8), or where they are grouped into four (like in 4/4).
Compound meter is used to descibe meter where beats can be divided by three (which means the beat will typically be assigned to a dotted value or a grouping of three). The most common example is 6/8. In 6/8 time, you have two groups of three eighth notes. Since it is a group of three, it is compound meter.
Just like with simple meter, you can have compound duple, compound triple, and compound quadruple.
The most common time signature is 4/4, so the phrase "common time" refers to this time signature.
You can feel meter in different groupings -- either small or large. If you were to feel the beat of every quarter note in 4/4 time, that would be common time. Now, if you were to feel the first and third quarter note in the measre (feeling two stronger beats per measure) you would be feeling it "in two." Feeling the beat this way turns 4/4 into 2/2 (two beats per measure, the half note getting the beat). You basically "cut" the 4/4 in half. This is cut time.
Sometimes a piece of music will have just one time signature (so just one meter) for the entire song. That's true for most pop music and marches. However, sometimes music can change time signatures throughout the work. For example, it might start in 4/4, then switch to 6/8, then to 3/4, and then back to 4/4. When a piece of music changes time signatures, it is referred to a "mixed meter."