http://en.wikipedia.org/wiki/Capo

lloyd, kay-po is correct.

http://dictionary.reference.com/browse/capo?qsrc=2888

chet atkins, mark o'connor, doc watson, etc used them. early 12-string guitar makers suggested to tune them to 'D so you will see gordon lightfoot, et al, with a capo on the 2nd fret. i like the sound of an acoustic guitar with a capo.