Deviations from the strict small integer ratios that compose the structural timing of a piece (Clarke 1999, p. 489). Expressive timing is introduced by a performer and certain types of expressive timing appear to be expected by modern musical audiences (490).
Expressive timing is remarkably consistent between different performances of a piece by the same performer, and even between different performers. It can be generated upon sight-reading and can be altered, to some extent, at will. Therefore, it is implausible that expressive timing is learned separately for each piece; Clarke asserts that it must be generated based on the structure of the piece (492).