Guide tones are the 3rd and 7th of a chord. They are what determine the quality of a chord (whether it’s Major, minor, or Dominant). The root, and 5th (for normal chords) always stay the same, and are thus less important.
e.g.
| Chord | Root | 3rd | 5th | 7th |
|---|---|---|---|---|
| CMaj7 | C | E | G | B |
| C7 | C | E | G | Bb |
| Cm7 | C | Eb | G | Bb |
| CmMaj7 | C | Eb | G | B |