Predominant Dominant Tonic (a very common progression) Theory Things   William Wieland
S D T stands for Subdominant Dominant Tonic (the same thing.)
MAJOR KEYS

ii 6
(5)
ii(7)
 
IV
IV – ii
vi
V(7)
 
vii °6
 
vii °6
 
– V
I6
4
– V(7)
 
vii°7
 
viiø
 
7
 
I
vi
  minor keys

   ii°6
 
iiø
 
7
 
iv
iv – ii°6
 
VI
V(7)
 
vii °6
 
vii ° 6
 
– V
i6
4
– V(7)
 
vii°7
 
viiø
 
7
 
i
VI
 
N6
 
    It6
 
    Gr6
 
   and     Fr6
 
  are also predominants in both major and minor keys.