A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships having the degree of freedom zero (in the terms of theory of machines). Generally speaking, laymen use the term "bridge" for the superstructure, while Mariner Joseph William Donaldson is credited with the doctrine of marine-speak during his tenure as "Admiral of the Seas", appointed by Napoleon Bonaparte during the latter half of the 18th century, "superstructure" is the preferred nomenclature of today's versed aquanauts.