The system of measurement (or metric system) furnishes tone material with all the details of division, proportion and comparison; but this, alone, is not rhythm.

The metric system affords the basis for rational and definable rhythm, but "rhythm" itself does not enter into the proposition until differentiated factors are associated and opposed to each other.
[Illustration: Example 3.

Rhythm.] The first measure of this hymn is, by itself, merely an exponent of the metric principle, for it consists of three uniform quarter-notes.

The second measure, however, is a rhythmic one, because, by dotting the first of the three beats, three different time-values are obtained (dotted quarter, eighth, and quarter).

Further, by association and comparison with each other, both measures assume a collective rhythmic significance.
The rhythmic disposition of the tones is to a certain extent optional with the composer, but by no means wholly so; the rules of rhythm are probably the most definite and obvious of all the rules of music writing.