In the kind of printing which involves setting type, a matrix (often called a "mat") is a mold for shaping the letters -- the mats of all the letters to go on one page are assembled, and then hot metal is poured into that matrix to make the plate to go into the printing press to print the page.

• The U.S. Congress, in enacting the Federal Rules of Evidence, used the term in that sense to define a "duplicate" of a document that can be used as evidence at trial in place of the original:

"A 'duplicate' is a counterpart produced by the same impression as the original, or from the same matrix." (Rule 1001(4))