Music Theory for the 21st-Century Classroom

Because period form involves “more conclusive” and “less conclusive” cadences, it is important to distinguish between inconclusive and conclusive cadences. Conclusive cadences end on the tonic chord, while inconclusive cadences do not.

In addition, the perfect authentic cadence (PAC) is more conclusive than the imperfect authentic cadence (IAC). While the plagal cadence (PC) occurs less frequently than the other three cadences listed in the table above, it will sometimes occur in root position at the end of a phrase after an inverted imperfect authentic cadence (IAC) has concluded a previous phrase, with the understanding that a root position PC could be considered more conclusive than an inverted IAC.

Generally, a period will either contain a phrase ending in a half cadence (HC) followed by a phrase ending in an authentic cadence (IAC or PAC), or it will contain a phrase ending in an IAC following by a phrase ending in a PAC.

In the first example, a HC concludes the first phrase and a PAC concludes the second phrase, making a period.

Here is a formal diagram of the above example.

Notice that one or more phrases within a period can be a sentence, as in the example above, since sentences are phrases with specific melodic structure.

In the next example, the first phrase concludes with an IAC and the second phrase concludes with a PAC.

In the next example, the first phrase ends with an IAC and is followed by a second phrase ending with a PAC.

Another example with a less conclusive cadence followed by a more conclusive cadence is “Lean on Me” by Bill Withers.

In a period, the phrase ending with the less conclusive cadence is called the “antecedent” and the phrase ending with the more conclusive cadence is called the “consequent.” These can be thought of as being in a “question and answer” relationship.

Periods are labeled as “parallel” or “contrasting” based on the melodic material. In a parallel period, the melodies in both phrases begin similarly. In a contrasting period, the phrases begin differently.

The three preceding examples are parallel periods

The apostrophe mark (’) is called “prime” and is used to show a phrase is similar to a previous phrase but ends with a different cadence. Therefore a’ is called “a prime” and a’’ is called “a double prime.” If you are analyzing a piece that requires triple and quadruple primes, it is clearer to use \(\left.\text{a}^{1}\right.\), \(\left.\text{a}^{2}\right.\), \(\left.\text{a}^{3}\right.\), and so on.

Below is an example of a contrasting period.

If you encounter a section consisting of the same phrase occurring twice, call it a “repeated phrase.”

In following two sections, we will examine periods containing more than two phrases.