The root relationship of the IV and V are perfect 5ths from the I (Tonic). The dominant is a 5th above the tonic and the subdominant is a fifth below the tonic. These three provide structure in building progressions.
Basic opening progressions are I-V, I-IV, or I-IV-V.
Basic closing progressions are: V-I, IV-I, and IV-V-I. Progressions of harmonic phrases are completed or closed by returning to the tonic.