The B major seventh chord is known as the B7 guitar chord. The B7 chord uses five strings.
The illustration on the left is for right handed players, and the diagram on the right is for left handed players.
This chord may take a bit longer to learn. A lot of blues is played in E, using E-A7-B7. Practice the B7 chord until you can get your fingers there quickly.
Switch back and forth between E and B7, then A7 and B7.
Try this basic twelve bar blues pattern. The "/" sign means: strum the chord one time towards the floor(not up and down), the letter shows when to switch to the next chord. Example: E/// is the same as EEEE. Keep a steady beat and count 1, 2, 3, 4, 1, 2, 3, 4, etc.
E/// //// //// //// A7/// //// E/// //// B7/// A7/// E/// B7/// then repeat. Have some fun with the twelve bar blues in E, until you have it memorized.
After you learn three more easy chords, you'll have learned the sixteen basic guitar chords, and will be able to play millions of sings using them!