What is "i-iv-v-i"

In music, I–IV–V–I or IV-V-I is a common chord progression and cadence that, "unequivocally defines the point of origin and the total system, the key."

Composers often begin pieces with this progression as an exposition of the tonality:

It may be viewed as an abbreviated circle progression:

I-IV- V-I = I–IV–V–I

"Along with motion toward the fifth (V), IV [the subdominant] appears as a corrective, depriving V (the dominant) of its independence and pointing it back in the direction of its origin [I]." In the key of C, IV provides the note F and eliminates the possibility of G major, which requires F. The progression is also often used at the end of works and sections.