The capo is a small but powerful tool for guitarists that raises the notes of the open strings but retains the individual relationships between strings. Here's how to use one.
A full capo – the most common type – is a clamp placed around your guitar’s neck to shorten all the strings, allowing you to sing and play in different keys. A partial capo shortens one or more strings, allowing you to explore alternative tunings without having to retune. Here’s a good introduction by folk singer and capo expert Harvey Reid.