A master mariner is a licensed mariner who holds the highest grade of seafarer qualification; namely, an unlimited master's license. Such a license is labelled unlimited because it has no limits on the tonnage, power, or geographic location of the vessel that the holder of the license is allowed to serve upon. A master mariner would therefore be allowed to serve as the master of a merchant ship of any size, of any type, operating anywhere in the world, and it reflects the highest level of professional qualification amongst mariners and deck officers.