• A time signature is two numbers, one above the other, at the start of a piece, after the clef and any key signature:
• The top number gives the number of beats per bar, so this is the number you need to count to before you start, and for each bar;
• The bottom number gives the length of each beat - 4 means crotchet (quarter-note).

• So in this example, there are three crotchet beats per bar:
• Up to now we have only seen crotchets (quarter-notes) as the length of the beat, which is "4" at the bottom of a time signature:
• You may see a "2" at the bottom, meaning the beats are minims (half-notes);
• "2" or "4" at the bottom is called "simple time" when each beat can be divided into two - minims into crotchets or crotchets into quavers;
• Or you may see "8" at the bottom - then there will nearly always be "6" or "9" at the top - this is called "compound time";
Hear it
• In compound time, there are two "levels" of beat:
• the quavers (the "8" at the bottom), so in this example there are 6 per bar, but they probably go quite fast;
• the quavers are grouped into threes - so each bar consists of two beats each made up of three quavers;
• To count in compound time, you normally count the higher level beats, in this example 2 per bar.

• A time signature is very important in sight-singing:
• You should always look at it before singing;
• You then know how many to count to in each bar;
• And how to relate the notes to each count.

• The time signature is not repeated at the beginning of each new line like the key signature but stays ths same throughout a piece of music, unless a new time signature is written:
• A change of time signature is written immediately after a bar line and then applies to the new bar onwards;
• If a change of time signature happens to occur at the end of a line, the new time signature is normally written at the end of the line after a bar line and then repeated at the start of the next line;
• If the top number changes but not the bottom number, it is normally assumed that the speed of the beats stays the same (unless the composer indicates otherwise);
• If the bottom number changes, the composer should normally indicate what speed the new beats should be relative to the old beats;
• This is normally done by giving two equivalent notes lengths, new = old;
• So, for example, if the time signature changes from 2/4 to 6/8 and is written over the change, it indicates that the speed of a dotted crotchet in the new time is equal to a crotchet in the old time.

For more details, see Basic Music Theory - Time Signature