X: a mark as a form of signature upon and acceptance of a contract by a person otherwise illiterate or physically unable to sign.
Also, an indication of dimensional measurement.
In an 1886 Indiana case, Jaqua v Witham & Anderson Co. (106 Ind. 547), at issue was the import of the symbols "3x5" in a lumber dispute. The Court acceptd the evidence in regards to the custom of using X to denote dimensions:
"That, by the usages and customs of the dressed lumber trade, the “3x5” ... meant either 3 feet by 5 feet, or 3 inches by 5 inches."