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An X mark is used to indicate the concept of negation as well as an indicator. Its opposite is often considered to be the check mark or tick. In Japanese, the X mark is called "batsu" and can be expressed by someone by crossing their arms.
It is also used as a replacement for a signature for a person who is blind or illiterate and thus cannot write their name. Typically, the writing of an X used for this purpose must be witnessed to be valid.
As a verb, to X off/out or to cross off/out means to add such a mark. It is quite common, especially on printed forms and document, for there to be squares in which to place x marks, or interchangeably checks.
It is also traditionally used on maps to indicate locations, most famously on treasure maps. It is also used as a set of three to mark jugs of moonshine for having completed all distillation steps, while additionally signifying its potency relative to legal spirits, which rarely exceed 80 proof.