Watch the Whipping Post online guitar lesson by Andy Aledort from Kings of Rock & Roll Vol. 3: Duane Allman
The Allman Brothers were greatly influenced by the modal jazz movement of the mid-sixties, as exemplified by the work of Miles Davis and John Coltrane, and "Whipping Post" is one of the finest examples of how the Brothers used this influence in a creatively artistic way.
This is an interesting groove to solo over: it's played in a 12/8 time signature (three evenly divided eighth notes per each of the four beats in the bar) with a chord progression that basically stays on Am the entire time. Duane likes to utilize the A Dorian mode (A B C D E F# G) on this tune, as I have focused on here, and also creates unusual rhythms by breaking up the eighth-note triplets into 16th-eighth-eighth-16th patterns.