Tonic – is the first scale degree / chord of a Diatonic scale (the first note of a scale) and the tonal center or final resolution chord of Tonic/Dominant harmony.

A Partial Diatonic, V of IV Secondary Dominant chord to temporarily tonize the next chord. Also used as a pivot chord for an Internal Modulation to new key/tonal center.

A true Diminished linking chord typically used between the I and II .

Or ♭II°7 is not a true Diminished chord.

It's actually a Disguised V of VI Secondary Dominant chord, a VI7b9 .

A notational shortcut vs. using the correct slash chord notation. Example: C♯°7 vs. A7♭9/C♯ . This can often be a publisher's marketing decision based on the perceived ability and level of their customers. Or, I think they just don't have good music editors.

An Enharmonic Equivalent for the #dim7 above.

Minor Third Substitution , A tritone sub for V , Usually precedes the I , a common Chromatic harmonic move.

Supertonic – functions as the subdominant of I , replacement substitute for IV .

V of V, Secondary Dominant . A Secondary Dominant chords are a Partial Diatonic pivot chord and often used for an Internal Modulation to a new key.

An Enharmonic Equivalent for the ♭III°7 below.

Not a true Diminished chord. A VII7b9 , Disguised V of III Secondary Dominant . A notational shortcut vs. using the correct slash chord notation. Example: D♯°7 vs. B7♭9/D♯ .

Minor Third Substitution , A tritone sub for VI , Usually precedes the II , a common Chromatic harmonic move.

Mediant, replacement substitute for I .

V of VI Partial Diatonic, Secondary Dominant chord to temporarily tonize the next chord. Also used as a pivot chord for an Internal Modulation to new key/tonal center.

Subdominant

m7 or m6 – The II of the bVII7 below . A Minor Third Substitution substitution for a V chord. Or, a mis-named ninth chord a fourth up or fifth below.

Partial Diatonic chord found in a Blues progression.

Not a true Diminished chord. A II7b9 , Disguised V of VI Secondary Dominant . A notational shortcut vs. using the correct slash chord notation. Example: F♯°7 vs. E7♭9/G♯ .

An Enharmonic Equivalent for the #IVdim7 above.

Dominant

The II of the V of IV .

Not a true Diminished chord. A II7b9 , Disguised V of V Secondary Dominant . A notational shortcut vs. using the correct slash chord notation. Example: G♯°7 vs. D7♭9/C♯ .

Substitute for IV usually leads to V .

Submediant – substitute for I , often follows I or occurs between III and II .

A Minor Third Substitution substitution. A sub for V leading back to a I . Typically used as IVm7 bVII7 to the I — coming back to the tonic using the backdoor and not via the dominant, front door, V .

Leading Tone – substitute for V , often follows I or occurs between III and II .