For instance, in No.
42 the cadences do _not_ fall in the 4th, 8th, 12th measures--and so on--but in the 5th, 9th, 13th, 17th, from the very beginning of the piece.
When the introductory passage is longer than _three_ measures, it probably constitutes a complete phrase by itself, with its own cadence; in which case, of course, it must not be analyzed as "extension." For example, at the beginning of No.

29; still more apparently at the beginning of No.

28, No.

41, and others.
* * * * * * INHERENT IRREGULARITY .-- Finally,--there exists another, third, condition, besides those mentioned at the head of this chapter, whereby a phrase may assume an irregular dimension; not by doubling or dividing its length (as in the large and small phrases) nor by the processes of extension,--but by an arbitrary and apparently incalculable act of _melodic liberty_,--by allowing the melody to choose its own time for the cadential interruption.

This comparatively rare occurrence is illustrated in Ex.
17, No.