Primary triad

Functional harmony tends to rely heavily on the primary triads: triads built on the tonic, subdominant, and dominant degrees.

[1] The roots of these triads begin on the first, fourth, and fifth degrees (respectively) of the diatonic scale, otherwise symbolized: I, IV, and V (again, respectively).

Primary triads, "express function clearly and unambiguously.

"[1] The other triads of the diatonic key include the supertonic, mediant, sub-mediant, and leading-tone, whose roots begin on the second, third, sixth, and seventh degrees (respectively) of the diatonic scale, otherwise symbolized: ii, iii, vi, and viio (again, respectively).

Alternatively, secondary triads may be considered ii, iii, and vi.