What happens when you change the time signature

When you change the time signature while the music is playing you must indicate how this change will effect the tempo.

Usually you will want to change the tempo beat for beat.

Here is how you do it.

Example 1

In this example we have set the tempo to 100 beats per minute. In common time the beat is a crotchet.

When the time signature changes to cut common time, we still want 100 beats per minute. Cut common time looks the same as common time, but there are two beats to the bar in cut common time, four beats to the bar in common time.

Think of a musician tapping their foot. From common to cut common it feels as though we have doubled the tempo. Two bars of cut common time make one bar of common time.

So we say to the musician “double tempo” and “crotchet equals minim”, and they know what we mean.

How long do these four bars play for?
Common time: (4 * 2 * 60)/100 = 4.8 seconds
Cut common time (2 * 2 * 60)/100 = 2.4 seconds
Total 7.2 seconds

Example 2

This exercise begins in cut common time and changes to common time. In effect, the instruction now is to halve tempo and minim equals crotchet.

How long do these four bars play for?
Cut common time: (2 * 2 * 60)/50 = 2.4 seconds
Cut common time (2 * 4 * 60)/50 = 4.8 seconds
Total 7.2 seconds

Example 3

From common time to compound time, beat for beat, we say l’istesso crotchet equals dotted crotchet. L’istesso is the official terminology for (beat for beat).

How long do these four bars play for?
Common time: (4 * 2 * 60)/80 = 6 seconds
Compound time (4 * 2 * 60)/80 = 6 seconds
Total 12 seconds

Example 4

How long do these four bars play for?
Compound time: (4 * 2 * 60)/54 = 8.9 seconds
Common time (4 * 2 * 60)/54 = 8.9 seconds
Total 17.8 seconds

Example 5

In summary, here are 18 measures of music to listen to, 7 changes of time signature, and constant 100 beats per minute from start to finish.

We start with a swing feel, 4 beats to the bar. In the third measure we are now clapping 3 beats to the bar. The music is no longer swung. It is now straight 8ths.

Four measures of cut common time play for the time of two measures of common time. A crotchet in common time equals a minim in cut common time.

In 6 – 8 time there is a natural accent twice as frequently as in 12 – 8 time.

Example 6

In examples 3 and 4 the value of the quaver changes with the time signature. In common time a quaver is half the beat. In compound time a quaver is a third of the beat.

Vary rarely, you might want to change the time signature from common to compound and keep the time value of the quaver.

We can no longer say Listesso. (Listesso means beat for beat.)

Can you work out the tempo change? At the start, there are 90 dotted crotchets per minute. That is 270 (3*90) quavers per minute. Maintaining quaver tempo, in common time crotchet equals 135 (270/2).

How long do these four bars play for?
Compound time: (4 * 2 * 60)/90 = 5.3 seconds
Common time (4 * 2 * 60)/135 = 3.5 seconds
Total 8.8 seconds