The time signature of a piece of music is shown by the two numbers that appear at the start. The most common time signatures you will come across, especially if you are a beginner, are 2/4, 3/4, and 4/4. The top number tells us how many beats there are in each bar, whilst the bottom number tells us what sort of note it is that we are counting.
When we start learning, we are told that a crotchet is a 1 beat note, a minim 2 beats and a quaver half a beat,
However, this only works in time signatures that end in 4 (2/4, 3/4 etc). These are termed simple time signatures. In other time signatures, we count different note lengths and this is what the bottom number signifies.
- 2 = minims
- 4 = crotchets
- 8 = quavers
- 16 = semiquavers
So, a time signature of 4/4 means we are counting 4 crotchets in each bar, but a time signature of 6/8 means we are counting 6 quavers. This means that a crotchet is only really a one beat note in a simple time signature, but in a time signature of 6/8, it would actually be 2 beats.
The Important thing is that the ratio between the notes never changes. There are always 2 quavers to a crotchet, and 2 crotchets to a minim.
Compound time signatures
Time signatures with an 8 at the bottom are called compound time signatures. These are ones where we count quavers.
In 6/8 we have 6 quavers in each bar, but we could also have 6 quavers in a bar of 3/4, so what is the difference?
As we can see, it is all about how the notes are grouped. In 3/4, we have three pairs of quavers which we count as 1 and 2 and 3 and but in 6/8 we have two groups of three quavers which we count as 1 and a 2 and a.
This means that in 6/8 we have two beats in each bar, but that each beat has three quavers in it. This means that a crotchet is not a one beat note any more, but is actually only two thirds of a beat.
Here is a second version where I have changed the time signature to 12/8.