The B7 Guitar Chord (also known as the Dominant B 7th Chord) is often referred to as a Blues chord. Once you learn to play it, you will understand why. It just has a real bluesy sound to it. The B7 Chord is found by first locating the 1, 3 and 5 positions (B, D# and F#) of the B Major Scale: B C# D# E F# G# A# B, and then adding a flat 7 note. To locate the flat 7 note, we first find the 7th note of the B Major scale (A#) and then go back a 1/2 step to the A Note.
So the notes of the B7 Chord are: B, D#, F# and A
There are several different ways to play it, but I will demonstrate the easiest way in the following post.
Here is a key that will help you read the chart:
Everything on the above chart should be self explanatory, except for the last three in the third column. They may need a bit of explaining. The O, or open symbol, simply means that you do not press down on any notes on that string. The X means you do not strum that particular string. The Barre symbol means you need to barre that particular fret. When you barre a fret you are pressing down on multiple strings at the same time with one finger.