Beats In The Bar
The old joke goes that this is what musicians are called - beats in a bar.
It is roughly speaking the top number of a time signature e.g. 3 in 3/4 time . The bottom number is the default note length. In this example it is a Crotchet or quater note. So in this case there are three beats and each beat is a crotchet. Note that the number of beats can be made up by any combination of notes that add up to the value of 3 crotchets. This is fine for Simple Time Signatures (2/4, 3/4 and 4/4). Simple Time Signatures are divisible by 2 and the beat is not a Dotted Note.
With Compound Time Signatures, things are a little less straightforward. Firstly, the number of beats is not divisible by 2 which means that there is no direct indication of where the beat lies. Take the Jig time of 6/8 as an example. The basic note is a quaver (1/8). There are 6 of them in a bar. So far so good. Is the beat in two groups of three or three groups of two though? In other words, should it be played
The answer lies in whether it can be simplified further and the second grouping (three groups of 2) is exactly the same as 3/4 time with each crotchet replaced by 2 quavers (remember the number of beats can be made up by any combination of notes). The first grouping cannot be simplified further unless dotted notes are used and so it is meant to be played that way if it is written as 6/8. The same applies to 9/8 and 12/8 - all groups of three.