The capo is a device which is placed across the neck of the guitar (acting as a movable nut). It has 2 uses:
Capos come in various shapes and sizes.
The capo allows you to play the song in the same key, yet at the same time use easier (open) chords. Consider a turnaround in Eb:
If you place the capo on the third fret, the following chords can be played without changing the song's key.
If you have studied bar chords, you will notice that the capo is acting as a bar.
To work out which fret the capo must be placed on, simply count the number of semitones between the 'capo' key you have selected to change to.* For example C, as used in the previous example and the original key (that is Eb in the example). Hence C to Eb = 3 semitones, and therefore the capo must be placed on the third fret. Expanding upon point 2, consider a song in the key of C, using the turnaround progression:
A singer may decide that this key is unsuitable for his or her voice range and may wish to use the key of, say, Eb. The progression, transposed to Eb, will become:
Instead of changing to these new chord shapes (that is having to use bar chords), the guitarist may still play the C, Am, F and G chords, but must place the capo at the 3rd fret to do so.
Original key:
New key:
* Remember you are not actually changing key but merely the chord shapes, for ease of playing.