What key is eyes of the world in?
The song is played in the key of E major and begins with an extended one-chord vamp and groove on an Emaj7 chord, over which guitarist Jerry Garcia adds solo improvisations based on the E major scale (E F# G# A B C# D#).
What tuning is Behind Blue Eyes?
The song is in standard tuning, but the slightly sharp tuning might be a studio effect or the result of analog recording. In any case, following along with the original recording might sound a tad bit off unless you raise your overall tuning about 25 cents.
What is a Cadd9?
True to its name, the Cadd9 chord is a C chord with a 9th note “added” to the mix. It’s comprised of four notes: C, E, G and D. Notice that the D is the 9th note that gives the Cadd9 chord its extra flavor. Try playing a C chord and then play a C9 chord to hear that extra punch that ninth note delivers.