Guide tones — the third and seventh of each chord — serve as stable landing points in jazz guitar improvisation, and mapping the diatonic notes that surround each guide tone is a productive way to expand vocabulary without losing harmonic clarity. This post explains what the Functional Diagonal Approach (FDA) requires before that mapping is reliable: the function of the chord must be identified first, because a minor chord functioning as the 2nd, 3rd, or 6th degree of the key will present a different set of surrounding diatonic notes even when the chord shape is identical.