First, let's decode the terminology. A (or ii-V-I) is a three-chord progression that forms the harmonic backbone of countless jazz standards, from “All the Things You Are” to “Autumn Leaves.” In the key of C major, a 251 is Dm7 (ii) → G7 (V) → Cmaj7 (I).
To understand the importance of the Jamey Aebersold 251, you must understand harmonic tension and release. jamey aebersold 251