Nut

The nut of a guitar (or other stringed instrument) is a small strip or block of hard material forming a transition between the strings' playing length and the tuning machines on the headstock, or the tuning pegs in the pegbox at the upper end of the fingerboard. Along with the bridge, the nut defines the vibrating lengths of the open strings.

Nut on an acoustic guitar

Nut on a classical guitar

Nut on an electric guitar

The nut may be made of ebony, ivory, bone, or plastic, and is usually notched or grooved to maintain an even string spacing. The grooves are designed to lead the string from the fingerboard to the headstock or pegbox in a smooth curve in order to avoid damage to the strings or their windings.

Not all guitars have nuts as described: some guitars have nuts that are just string spacers, with deep notches. These instruments use what's called a "zero fret", which is a fret, at the beginning of the scale where a normal nut would be, that's higher than the other frets to provide the correct string clearance. This is usually found on cheaper instruments, as it's much easier to set up an instrument this way; to make a proper nut requires that each string notch be carefully cut to the proper depth so that the string is neither too high nor buzzes against the frets on account of being too low. With a zero fret, the "nut" fret merely needs to be the right height.