Irregular time signatures
So in 4/4, the music naturally groups into 4 crotchet beats, whilst in 6/8, the grouping would tend to fall into a pattern of 2 groups of 3 quavers (or 2 dotted crotchets).
Irregular time signatures occur when the music does not naturally fall into equal groupings.
5/4 where there are 5 crotchet beats in each bar is perhaps the most common of irregular time signatures. It is not possible to divide the grouping of notes equally within the bar. This results in an irregular grouping pattern of 3 and 2, or 2 and 3.
Listen particularly to the opening of 'Take 5' which is in 5/4 time. The music is grouped like the first example above – 123 + 12. This is especially clear in the first 20 seconds or so before the saxophone soloist enters.
In 'Money' the music is in 7/4. The grouping of the beats in this example is 1234 + 123.
An example of irregular time signatures in 'Take 5' by Dave Brubeck.
An example of music in 7/4.