One frequently encountered chord that causes prolongation is the nonfunctional subdominant. A nonfunctional subdominant is a IV or iv chord that appears between two tonic chords.

Recall that the subdominant chord typically has a predominant function, and therefore tends to go to a dominant-function chord such as V or vii°6. Occasionally, however, it moves to tonic, meaning that its usual predominant function is unobserved. Instead, it prolongs the tonic function, as shown in the example below. Notice that when it does not introduce its own function, the subdominant needs parentheses. When it does have a predominant function, it carries no parentheses.

I	(IV)	I	I6	IV	V	I	(IV)	I
T	[T]	[T]	[T]	P	D	T	[T]	[T]

When a nonfunctional subdominant appears between two I chords after an authentic cadence, it is often referred to as a plagal cadence. It is also sometimes called an "Amen" cadence due to its use in sacred music. The Plagal cadence, however, cannot actually end a phrase, and is therefore arguably not a "real" cadence. The "real" cadence is the authentic cadence that comes just before it.