A capo is a device that clamps down across the fingerboard at a particular fret on a guitar. Capos can operate by means of elastic, springs, or even threaded bolts, but they all serve the same purpose — they shorten the length of all the guitar strings at the same time, creating, in effect, a new nut. All the "open" strings now play in higher pitches than they do without the capo.
How much higher? A half step for each fret. If you place the capo at the 3rd fret, for example, the open E strings become Gs (three half steps higher in pitch than E). All the strings become correspondingly higher in pitch as well — B becomes D, D becomes F, and A becomes C. (By the way, you can't play anything below the capo — only above it on the neck.)
To correctly set the capo, place it just before the fret (toward the tuning pegs), not directly over the metal fret wire.
Why should you use a capo?
A capo enables you to instantly change the key of a song.
Say you know how to play "Farmer in the Dell" in the key of C and only in the key of C. But you want to accompany a singer (maybe yourself) whose vocal range is better suited for singing "Farmer in the Dell" in the key of D.
No problem. Put your capo at the 2nd fret and simply play the song in C as you normally do. The capo causes all the strings to sound two half-steps higher than normal, and the music sounds in D! In fact, you can move the capo to any fret, sliding it up and down the neck, until you find the fret (key) that's perfect for your vocal range.
Of course, if the notes and chords in the song you're playing have no open strings, you can simply change positions on the neck (using movable chords) to find the best key for singing. Use a capo only if the song requires the use of open strings.
A capo gives the guitar a brighter sound.
Just place a capo on the neck (especially high on the neck). The guitar will sound more like a mandolin (you know, that teardrop-shaped little stringed instrument that you hear gondoliers play in films set in Italy).
Capos can prove especially useful if you have two guitarists playing a song together. One can play the chords without a capo — in the key of C, for example. The other guitarist can play the chords in, say, the key of G with a capo at the 5th fret, sounding in C. The difference in timbre (that is, the tone color or the quality of the sound) between the two instruments creates a striking effect.
A capo allows you to move, to any key, certain open-string/fretted-string combinations that exist in only one key.
Some people refer to capos as "cheaters." They think that if you're a beginner who can play only in easy keys (A and D, for example), you need to "cheat" by using a capo to play in more difficult keys. After all, if you're worth your salt as a guitarist, you could play in, say, B flat without a capo by using barre chords.
But in folk-guitar playing, the combination of open strings and fretted ones is the essence of the style. Sometimes these open-string/fretted-note combinations can become quite intricate.
Think, for example, of the introduction to "Fire and Rain," by James Taylor, which he fingers in the key of A. James plays it, however, by using a capo at the 3rd fret, causing the music to sound three half-steps higher, in C, because that key best fits his vocal range. So why not just play the song in C without a capo? Because the fingering makes that option impossible; the necessary open strings that James plays don't exist in C — only in A!
A capo "moves" the frets closer together as you go up the neck.
Playing with a capo requires less stretching in the left hand, making some songs a little easier to play.
Any halfway serious guitarist should get a capo and experiment with it. See how you can use a capo to find the best key for your vocal range. Place it at various frets to see how that placement affects the guitar's timbre. You're sure to like what you hear.
