Jazz performers often improvise rhythms that are not written out in the score. These rhythms are based on years of experience and practice playing several scales, complex rhythmic passages, intuition, careful listening and responding to what the ensemble plays. This takes a great deal of practice and experience performing within the group. Generally, the drummer and ensemble will comp chords with a steady beat, allowing the soloist to keep time and improvise upon the chord changes.
Aleatoric music appears in jazz often. Aleatoric music, commonly known as chance music. In aleatoric music, the performers are given specific rules to follow in a non-defined random order. Regarding rhythm, aleatoric music can be any combination of rhythms that are designed to be played in tempo, or out of tempo. The important thing with aleatoric rhythms is that some pattern or structure emerges to keep the rhythms from becoming too vague and unrecognizable.
Sequences can be another type of unmeasured rhythms. Often in jazz music, a performer will take a small fragment of a rhythm and repeat it in such a way that it creates an unmeasured, improvisatory rhythmic motive. These sequences are then transformed into new rhythms, which are loosely connected to the original rhythm. A skilled jazz player will be able to create unlimited rhythmic combinations based on the initial sequence.
In a lead sheet, the performer is given no required rhythms to perform. The performer will use the chord symbols in the lead sheet and create melodies based on the progression of chords. This type of unmeasured rhythm occurs commonly in jazz music. Performers have to creatively think about ways to create melodic ideas based on the tones available in each chord symbol.