This is an absolutely essential topic to at least know the basics of as it is the basis of all things rhythmic. Learn what the two numbers mean and what you will and won't see in a time signature.
When written down (or notated), music is split up into smaller sections called bars. In each bar there are a set number of beats. This number is determined by the time signature. For example, a lot of music is written so that every bar has four crotchet beats. In such a piece the time signature would be:
The top '4' represents the number of beats per bar: four. The bottom '4' tells us what kind of note is counted. In this case it's crotchets (or quarter notes, hence the 4). The only numbers you will ever find on the bottom are 4, 8 (meaning quavers, or 8th notes) and 16 (meaning semiquavers, or 16th notes).
So as a second example the time signature of 'six eight' would contain six quavers.
There are certain rules that need to be followed when notating pieces of music that are dictated by the time signature. First of all bars must always be complete and you cannot exceed the number of beats allowed in the bar. For example:
This would be wrong as there are 5 crotchet beats in this bar when the time signature says there should only be 4.
The way certain groups of notes are beamed or (joined together) is also affected by the time signature. For example in time signatures with a 4 at the bottom, eighth notes are beamed in twos or fours and sixteenth notes are joined in fours. E.g.:
a) is correct as the eight quavers are grouped in two lots of 4.
In time signatures with an 8 at the bottom, quavers are beamed in 3’s and semiquavers in 6’s. When working out what time signature a written piece of music is in, this is a big give away.
The time signature is always found at the start of a piece and means that bar and every bar after it are in that time signature. You will only find another if the time signature changes.
You will sometimes see a C in the space you would expect the time signature to be. This stands for 'common time' and basically means 'four four'.
This is a quick introduction to the basics of time signatures. There is a lot more theory relating to this topic that will be covered as you progress through these information sheets.