Capos are sometimes thought of, even referred to as, cheaters. Nothing could be further from the truth for the bluegrass, country or folk guitarist.
There's a good reason guitarists love certain keys and would prefer to avoid others. For the same reason brass players often hate the guitarist's favorite keys - and love those that their guitar picking friends detest. Neither of them is showing a lazy attitude, it's that every instrument has keys that are just plain easier to play than others. The other point about these specific keys is that they actually sound better for reasons we'll get to shortly. It's when the two kinds of instruments meet that the guitarist's most useful small accessory often comes out - the capo.
Give me a G
Short for capotasto Italian for "head of fretboard" it's like a movable nut for the guitar. No matter what the individual design (and there are some very ingenious ones) the capo, in essence, clamps the strings to the fretboard at a position between the nut and the high end of the fretboard, shortening the vibrating length of the strings and raising the pitch of whatever chord the guitarist is playing by however many frets up the neck he's placed his appliance. It sure sounds like cheating and some jazz or classical players would never use one, but folk, country, bluegrass and blues find their capos very useful indeed. Taking the example of bluegrass - many instruments essential to the genre are well suited to the key of G in particular. The banjo is tuned to an open G variation and so is the dobro so you'll find that players of these instruments have an inordinate love of the key of G. But bluegrass is not solely instrumental music and, as with any vocal music, the needs of the vocalist come first. If he/she wants to sing in Bb, Bb it is. Out the window go all those open string runs, hammering on and off on the guitar. Or do they? Bluegrass guitarists (dobroists and banjo players too) will pull a capo out of their pockets and place it on the third fret. They'll play chord shapes in the key of G with all the open string advantages of that key, but they'll be pitched in Bb.
The added advantages for the guitarist
We've already mentioned the use of open strings that is facilitated by playing G shapes capoed up to Bb but there are other peculiarities of particular chord shapes that make guitarists prefer them. The D chord is great for jangly Byrds type melodies, so is A. Using D shapes a guitarist often drops his low E string down to a D, giving a very full bodied sound to the accompaniment. Using a capo can extend the usefulness of the "dropped D" tuning. In addition, as the sounds of the guitar strings become higher and higher, the instrument itself begins to take on some of the characteristics of smaller bodied, higher pitched instruments such as the mandolin and this can make for a very interesting arrangement, particularly when combined with another guitar capoed lower or not capoed at all.
Here comes the capo
If you need any more convincing on the worth of the capo, have a closer listen to the chiming sounds of George Harrison's guitar in Here Comes The Sun - played with D shapes capoed on the seventh fret. It just wouldn't sound the same done any other way, no matter how good a guitarist you might be.
