On pages 106-107 of "Life", Keith explains how Jimmy Reed played a V chord (B 7th) in an E blues progression. He says Jimmy left the A string open and slid his finger up the D string to a "7th." Does anybody understand how this works? What note is he saying to play on the D string? Do you continue playing the open A, or do you just let it ring? I can't find a note on the D string that sounds right (i.e, like a B 7th chord) against the open A string. Thanks!