In a word, there is no doubt of the unbroken connection of these three phrases, despite the unusual weight of the first cadence.

See also the first cadence in Ex.

51.
By simply continuing the process of addition (and avoiding a decisive perfect cadence) the phrase-group may be extended to more than three phrases, though this is not common.
THE DOUBLE-PERIOD .-- A third method consists in expanding the period into a double-period (precisely as the phrase was lengthened into a double-phrase, or period), _by avoiding a perfect cadence at the end of the second phrase_, and adding another pair of phrases to balance the first pair.

It thus embraces four _coherent_ phrases, with a total length of sixteen measures (when regular and unextended).
An important feature of the double-period is that the second period usually resembles the first one very closely, at least in its first members.

That is, the second phrase contrasts with the first; _the third corroborates the first_; and the fourth either resembles the second, or contrasts with all three preceding phrases.