|Simulating open tunings
A very special way of using the capo is partial capoing. If you are experimenting with open tunings, this might be the thing for you:
There are capos that allow you to capo the strings individually. One or more such capos can form a chord. Thus the sound of open tunings can be simulated - without retuning. The advantage is that all scales and closed chords stay the same, but at the same time the sound of the open tuning is available. It saves you the trouble of figuring out all the new chords and scales. On stage, partial capoing may be more practical than retuning or bringing an extra instrument.
This type of capoing is also great for simplifying the fingerings for beginners. You can actually fret fullsounding chords with only one finger!
The simplest form of partial capoing on a guitar, is to put a capo on the second fret, but leave the sixth string (low E) free. Now you can play in the key of E using D-chords, and you have the deep bass note on the sixth string as if you had tuned it down. You get the sound of the drop-D tuning, but in the key of E. Note that you don't have to change the fingering on the sixth string from what you are used to. Kyser makes a capo for this purpose.
A common way of partially capoing the guitar, is to emulate the DADGAD tuning: Capo the second fret of the third, fourth and fifth strings and leave the other strings open. This makes an Esus chord. Shubb makes a capo for this purpose.
By placing a regular capo behind a partial capo, you can move the configuration to different keys.
More information on partial capoing: Third Hand Capo and Shubb.
Randall Williams teaches you more on his DVD: Partial Capo Techniques.