Normally at the end of a phrase we find some type of cadence. If chord V (or V7) is used as the penultimate chord, the final chord of the phrase will usually be chord I, making a perfect cadence.
Occasionally though, other chords may be found in place of chord I, and the most likely would be chord vi.
The cadence V-vi (or V-VI in minor keys) is called an interrupted cadence. It normally sounds a little bit surprising, because our brains are conditioned to expect chord I after chord V. Composers can take advantage of this, and use it to extend a piece of music by first writing an interrupted cadence, then continuing for a little while longer, before the expected perfect cadence arrives.
This (edited) example is taken from Mozart’s Piano Sonata no.3, K281 (2nd movement).
In SATB (four-part harmony) the third of the chord is doubled in chord VI, when this cadence occurs in a minor key. This is to avoid breaking other rules of harmony.
Here is another example, from Bach’s Chorale no.196. Interrupted cadences are relatively rare in Bach Chorales.
