Updated: Aug 11, 2019
The main features of a “period” theme are:
Weak middle cadence
Returning basic idea (instead of repeated)
When I mention the latter as “main features”, I mean: “these are facts that differentiate the period from the sentence”.
The period as a formal type contains only two functions/phrases:
The Antecedent is built up of:
2 m. Basic idea: the same as in the sentence
2 m. Contrasting idea: -particular to the period- can share some characteristics with the continuation function of the sentence
The Consequent phrase brings a return of the “b.i.” followed by a new contrasting idea which might not resemble the one on the antecedent. The consequent characteristically and univocally ends on a PAC and very rarely on a IAC.
Exactly the same as in the sentence with the difference that instead of being immediately repeated, it returns at the beginning of the consequent.
The contrast is achieved by means of melodic-motivic change. The contrasting idea might even introduce brand new motives. The supporting harmonic progressions can also be used as a mean of contrast. The contrast can be subtle or radical.
If the contrasting idea is supported in full by a cadential progression then it can be called "cadential idea", the same as in the sentence.
Weak Cadential Closure
The partial closure - the ending of the antecedent - should always be supported by an HC, or more rarely, by an IAC -it can never be a PAC-
Cadence Categorization - Strongest to Weakest
Return of the b.i.
The return of the basic idea behaves in the same way the repetition of the basic idea does in the sentence (Exact repetition, statement-response or sequential).
Strong Cadential Closure
The consequent phrase most often ends in a PAC and rarely on an IAC. The cadential progression will start earlier than in the antecedent for the obvious reason that the last chord is the “I” instead of the “V”. This means, the progression needs an extra chord to conclude and therefore needs to start sooner.
It is a brief musical idea that links antecedent and consequent in order to keep the flow. Conceptually the lead-in could be eliminated.
An elision happens when the end of one formula unit directly coincides with the beginning of the subsequent unit.
Even when the antecedent seems to elide with the consequent, this is just a superficial impression. The classical repertoire proves the later is just a misperception. The asymmetry that could arise makes this analysis unviable.
Ends and Stops
An “end” requires a cadence while a “stop” implies a discontinuation of the musical stream.
The mini sentence arises when the basic idea is built up of two 1 m. motives. The mini-sentences normally come in pairs.
Modulating Period - Cadential Strengths
In the majority of cases the period will start and end on the same key. Regardless of the keys in question, the cadences retain the syntactical strength corresponding to their types.
Differences and similarities: Period Sentence
Both themes are normatively eight measures in length, divided into two 4-m. phrases.
Both themes end with a cadence
Both themes begin with a 2 m. basic idea which is restated later in the theme
The sentence has one cadence; the period has two cadences.
The sentence can end with any cadence type (PAC, IAC, or HC); the period cannot end with an HC.
In the sentence, the basic idea is repeated within the presentation phrase; in the period, the basic idea returns at the start of the consequent phrase.
The sentence motivically pushes forward while the period reinforces a sense of stability.