The guitar nut

A guitar nut is a small piece of material that lifts the strings off the top of the guitar face. It fits between the headstock and the frets. The guitar nut sits at the opposite end of the guitar saddle.

The nut is placed at the end of the fingerboard and controls the string spacing across the neck and the proper height from the fingerboard. The nut may be made from ebony, ivory, cow bone, brass, corian (hard white plastic material) or plastic and is usually notched or grooved for the strings. 

To help you remember that the nut is at the end of the fingerboard recall the following image.

The nut was at the end of her finger and was stuck because she screwed it on when she was bored. (Nut at the end of the fingerboard).