No. Guide tones for sevenths will still be the third and seventh. Of course, other chord tones lead and color (Fifth, Ninth, Eleventh, Thriteenth), but the third and seventh define the chord's character. The additional diatonic or color tones (2,4,6 + 9, 11, 13) and/or altered ones b/#5, b/#9 and #11 paint the chords with more color and intensity.
When the term guide tone is used, it is such when finding, building the appropriate tritone sub (The Augmented Fourth/Diminished Fifth or three whole tones away in either direction) which often happens on dominant sevenths, but can also happen on altered dominant sevenths and/or ninths.
F#7 for C7 = the guide tones are E and Bb. The distance between these two notes is a tritone. When such a sub is evoked, what were the guide tones in the original chord are flipped. The third becomes the seventh and the seventh becomes the third: C-E-G-Bb ---> F#-A#-C#-E or C-E-G-Bb ---> Gb-Bb-Db-Fb (E))
1-3-5-7 ---> 1-3(7)-5-7(3); 1-3-5-7 ---> 1-3(7)-5-7(3) C7 ---> Gb/F#7
7b9s consist of two tritones: C7b9: C-E-G-Bb-Db: E-Bb (3 and 7) G-Db (5 and b9)
The tritone sub for this chord is also a 7b9: Either Gb or F#
With ninths, the third and seventh still flip, but the fifth becomes the tritone sub's b9 while the tritone sub's b9 becomes the fifth: C-E-G-Bb-Db ---> F#-A#-C#-E-G and/or Gb-Bb-Db-Fb-Abb (and I know Abb is G)
1-3-5-b7-b9 ---> 1-3(7)-5(b9)-7(3)-b9(5)
Dom7s with altered fifths (b5 and #5):
C7b5 = C-E-Gb-Bb - Tritone sub I = Gb-Bb-Dbb-Fb; Tritone sub II - F#-A#-C-E (F#/Gb7b5)
So, your guide tones are the third, fifth (altered) seventh and ninth (altered).
Tritone substitution allows for half slides or approach chords from a halfstep above or below the target chord. This is useful for walking chromatic basslines and easier/smoother Authentic cadences (ie: V-I ---> V-bV/V-I) Here it is in a ii-V-I: Dm7-G7-CMaj7 ---> Dm7-Db7-CMaj7 (B and F are in G7 and Db7)
If you hear those same two or three tones, you have substituted correctly. Here's a couple of YT video explaining them:
Halfstep Slides (Piano): http://www.youtube.com/watch?v=g9ji-WjVLjw&feature=g-user-u
F(add9)-B9-BbMaj9 as opposed to F(add9)-BbMaj9
Halfstep Slides/Tritone (b5) Subs: (Guitar): http://www.youtube.com/watch?v=moIf1x-79Zs
(Tritone Substitution used in intro piece)
You can have subs on m7s, but generally they're found on Dom7s (Major triads with the flatted 7th).
(I know, Jeff, but detailed explanations help, too and you beat me to it!)
post edited by Rus W - 2012/10/05 00:48:52