A what? You've probably seen them but didn't know what they were called. Or, you've seen them and never quite understood them. Maybe you've seen them, you think you know what they do, but you don't have it quite right. Either way, I'm here to help you out.
First off, here are a couple of capos:
A capo (pronounced kay-poh) is simply a mechanical device that acts as an extra finger. Some people have said that capos 'change keys', but this is somewhat of a misconception. While allowing the player to play in different keys is an effect of the capo, it is not its core function. It is important that one understands the distinction in order to fully understand a capo.
Okay, So What Does A Capo Do?
I think most guitar players of many different levels would agree that the following chord shapes are very common:
What do they all have in common? Yup, open strings. Try moving each of these chords up one fret and playing them. How do they sound? Not as pleasing as before, right? The reason they sound different is because your fingers moved but the open strings remained where they were, leading to different distances between notes. Wouldn't it be nice if the open strings could follow your fingers when they moved, especially if these were the only 3 chords you knew?
That's what a capo is for. If you were brave enough to even attempt to finger what used to be the open notes as you moved the chord shape up the neck, you've discovered that these chords are very cumbersome to play. All you have to do is slap on the capo where you want your open strings to be, and then play your old chords accordingly. The capo is now the lowest possible fret, your new fret zero.
This is why a capo is very popular with singer/songwriters and people who sing, because it easily assists with changing keys, so you can find that perfect key for your voice or the one you're singing with. And, if there are only 5 or 6 chords in your entire mental chord dictionary but you know how to use a capo, you can play just about anything you need to.
The tricky thing with capos is that you need to think in two disciplines at once, especially if you're playing with other instruments. One of those disciplines is probably the most familiar to this audience, what I call the "Guitar Land Key". The other is what is commonly called "concert pitch".
Guitar Land key is the chord shapes you are playing, ignoring the presence of the capo. For example, I may play the above chord shapes with a capo on 2 and say I was playing in the key of G. That would be guitar land key, because if you communicated this to a piano player, they would plunk out the notes and give you a funny look.
That's because the capo acts as an addition tool. The fret number that your capo is on is the number of half steps that is added to the guitar land key you're playing. For example, if your capo is on fret 2, you are adding 2 half steps (1 whole step) to every chord you play. The sum of this addition problem is the concert pitch key that you're playing in, and the key that you should communicate to other instruments that don't use capos.
Like I said before, a capo is like an extra finger. It's not a clamp, a vice, or a paper weight. So you want a capo that is going to apply just enough pressure to make the open strings sound good but not so much that it pulls your strings out of tune. The Kyser quick change capos put a billion pounds of pressure on your strings and pull them out of tune. I would not recommend them. My favorite guitar capos are made by Shubb and you can vary the amount of pressure that is applied to the strings by the capo. This is the guitar capo that I recommend to all of my guitar students. Plus, most neck thicknesses change from the first fret to the last fret-- it's essential that you have a capo that can adjust to it.
Another mistake that I see people making often is applying the capo incorrectly. Again, the capo is like an extra finger-- where do you place your fingers when you barre across the strings? It's not anything fancy, just across the strings parallel to the frets. You want to apply a capo in the same way.
Lastly, do not move or wiggle the capo once you clamp it down in position. Doing so will pull the strings in different directions and make them go out of tune.
A 'cut' capo is one that has had the pad part that contacts the strings cut away so that strings can pass under the strings unclamped. This can allow for all sorts of interesting alternate tunings. Though I have cut some of my old capos, I haven't experimented with it a whole lot because I don't like cutting up my capos. There is a guitar capo called the Spider Capo that will let you vary the pass-through points of the capo, no cutting required. I've heard good things about it but haven't tried it myself.