​SECONDARY TRIADS
The secondary triads are ii, iii, vi, and viio.  They are named the supertonic, mediant, submediant and leading tone.  
  • I           
  • ii             Supertonic
  • iii             Mediant
  • IV           
  • V                                                              
  • vi            Submediant
  • viio         Leading Tone                                                                    . 
 SUBSTITUTION​
​Each secondary triad is related to a primary triad through two common tones and thus can substitute for the primary triads.
You can substitute secondary triads for the following primary triads:
  • vi    for    I
  • ii     for    IV
  • iii or viio   for    V