Clipper guard
A plastic comb fixed to clippers that sets how much hair is left, numbered from short to long.
Also: guard number, grade
A clipper guard, or attachment comb, clips onto hair clippers and holds the blade a set distance from the scalp. In common systems a number 1 leaves about 3 millimetres of hair and a number 8 about 25 millimetres, with each step adding roughly an eighth of an inch. A number 0, or no guard, cuts closest. Asking for a grade, as British barbers say, is a quick way to describe a buzz cut or the sides of a cut.