Once you become more familiar with all these shapes, you'll find that you can begin to use bits and pieces of all of them and use them as moveable shapes, chunks of music that can be moved up and down the fretboard, changing pitch and name while still retaining their original flavor. This is the principle behind barre chords, but in that case you use your index finger to barre all six strings and become the nut while you move whole six-string shapes behind it. But you don't need to use all the notes of chords. Very often, you can use just three or four strings. This makes it much easier to find new positions for chords, since barring is always the most difficult and uncomfortable aspect of playing chords.
There is one more important open chord to mention: the open B7. It shows up often in tunes that are in the key of E, which is probably the most well loved of guitar keys, and this stand-alone shape is the most commonly used shape for this chord. I didn't include it in the first batch of open 7th chords because it has no plain old major counterpart. You can just learn the finger positions for it by looking at the diagram below, but I think it would be a good idea to watch the short movie above.
You'll see that it's really a C7th chord—which you learned earlier on—moved down in pitch one fret and tweaked a little bit. The more you can relate all these shapes to the original CAGED shapes the better. The quickest way to take the mystery out of guitar chords is to always see them as variations of those five original shapes. So, even though this B7th is a stand-alone chord, a one of a kind, it does in fact come from the C shape, even if it's not obvious at first.
The green dots show you where to put your finger tips. The red crosses mean 'Don't pluck/play this string'. The blue numbers indicate the best left hand fingers to use.
1 = index; 2 = middle; 3 = ring; 4 = pinkie.
The important thing to remember about this chord is that it's a 7th chord, made up of the same ingredients as any other 7th chord, namely the first (1), third (3), fifth (5) and flat seventh (b7) notes of its scale, and therefore has the same sound quality as any other seventh chord.