5-1 interval

Chords whose root notes relate by a fourth have a strong relationship, with the lower chord pulling towards the higher chord. This is because this relationship resembles that of the dominant chord to the tonic. The diatonic chords that fall into this pattern are as follows:

V-I

I-IV

ii-V

iii-vi

vi-ii

If you want your progressions to build, follow this flow. If you want them to open or relax, reverse the flow.

Mediants

Diatonic chords whose roots are a third apart share two of their three notes. Because of this, one can transition between these chords very smoothly. Following the same principle, moving between chords who do not share common tones will create a greater sense of movement.