Why Jazz Guitar Uses the 6th Degree on the One Chord

In bebop and swing jazz, the tonic I chord is almost always treated as a sixth chord, not a major seventh chord — a practice rooted directly in how Charlie Parker and the swing-era players heard and played the one. This post explains the historical and harmonic logic behind this choice, why George Benson's signature I-chord phrase is built around the 6th degree, and what it means for jazz guitar improvisation and the Functional Diagonal Approach (FDA).