Note:  Do not rely on this information. It is very old.

Mate

Mate, an old naval rank equivalent to the modern one of sub-lieutenant, in favour of which it was abolished in 1861. "Mate" in the navy now means "assistant," and is applied to various petty officers, as, for example, boatswain's mate, carpenter's mate. "Signal mate" is still the colloquial name for the sub-lieutenant to whom the management of a ship's signals is assigned. In the mercantile marine "mate" is the title of any officer who performs such duties as in a man-of-war would be performed by a lieutenant.