The I V vi IV progression

1. I

   Root note. The chord takes its name from this degree.

2. V

   Third. Major or minor quality is decided here.

3. vi

   Fifth. Stable, defines the chord body.

4. IV

   Color tone. Adds character on top of the triad.

In C major: C G Am F. Try in any key. Add a capo to change pitch without changing fingerings.