Watch the B7 Chord Pattern online guitar lesson by Stephen Mougin from Bluegrass Rhythm Evolution

On the B7 pattern, pluck the A string (B note), strum, move the middle finger to the 2nd fret of the E string (F# note) and pluck, then strum.

This almost always appears as the 5 chord in E. One important exception is the bluegrass classic "Old Home Place" where the B7 goes between the G and C chords. The B7 is a bit of a left hand crunch, so slow it down and practice making your hand get to that shape at once (rather than laying fingers one-at-a-time).