Guide Tones are a set of notes that outline voices in a chord or progression, usually spelling out what type of chord and descending in a linear fashion within the progression. The most commonly referred to Guide Tones are the 3 and 7 of a chord (often called The Guide Tones). Within most sub-genres of jazz it is standard to play chords with a 7 (major 7, minor 7 , dominant 7, etc.). Since a natural/perfect 5 appears in the most common chord types, such as -7, maj7, dom7 (and for mathematical reasons), it does not tell you much about the chord. The root of a chord is pretty essential to expressing the chord itself but not the most important thing for functional purposes. The 3 and 7 of a chord will tell you the most about what type of chord it is, hence being the most common Guide Tones. Note that the root note is usually played by the Bass player (or often left hand of the keyboard).

Circle of fifths progressions are the root of functional Jazz harmony. The II-V-I progression is a circle of fifths expression of the IV-V-I progression of Classical music. II replaces IV in the same fashion as a relative minor and creates the circle of fifths motion. VI-II-V-I is also very common in Jazz and is yet another extension of this motion. Using the standard Guide Tones, 3 and 7, outlining a circle of fifths chord progression, each of the two voices will either stay the same or move down by step. Also, the voice playing 3 on the first chord will then be playing 7 on the next chord and vise versa. For example, the 3 and 7 of each chord of a VI-II-V-I in C major:

• Chord : 3 , 7 A-7 : C , G D-7 : F , C G7 : B , F CM7 : E , B

You can see a smooth linear motion between two voices:

  1. Starting on 3 of A-7: C, C, B, B
  2. Starting on 7 of A-7: G, F, F, E

Other notes can be referred to as Guide Tones as well and perform a similar back and forth between two voices with common tones and motion by step. A common example is the 9 and 5. Notice that neither of these tones bears the same importance of chord quality.

Chord : 9 , 5
  A-7 : B , E
  D-7 : E , A
   G7 : A , D
  CM7 : D , G

Again, you can see a smooth linear motion between two voices:

  1. Starting on 9 of A-7: B, A, A, G
  2. Starting on 7 of A-7: E, E, D, D

The primary purpose of Guide Tones is to express the chord type. This is largely why they are so important for improvising. Being aware of the Guide Tones of each chord within the progression allows the improvisor to know what chord is being played, and with that, where they are in the form. Teachers often have students of all instruments practice singing the 3/7 Guide Tone lines to internalize them. While comping it is important to play the Guide Tones because they spell out the quality of the chord and that is what the soloing musician is basing their approach on, even if they are playing outside it is based on it being outside the changes they are expecting. Depending on what type of group you are comping for, you will be more or less encouraged to follow the soloist outside or to introduce harmonic substitutions not directly inspired by the soloist.

As is true with all music theory, these 'rules' are merely an expression of how to live out a specific style or genre of music and are ultimately more an explanation than actual rules.