Functional Harmonic Progression

The strongest chordal relationship is V - I. In the Baroque Period, composers began to develop this relationship and play chords in a certain order that led more and more strongly to the V - I cadence.

I - iii - vi - IV - ii - V - I

In a functional progression, chords are always played moving from left to right along this order. You can skip a chord, but you cannot go from right to left unless you go back to I. (The one exception is that V can go to vi. This is called deceptive motion.)