Bascule Bridge

A bascule bridge is a movable bridge whose deck, or leaf, is counterweighted and pivots upward on a horizontal hinge called a trunnion to let ship traffic pass beneath, its name taken from the French word for seesaw. Simple counterweighted drawbridges have medieval and even earlier antecedents, but the modern hydraulically or electrically powered bascule bridge became common from the late nineteenth century onward as growing rail and road traffic demanded reliable, quickly operable crossings over busy shipping channels. London's Tower Bridge, completed in 1894 with a pair of bascule leaves that can be raised in about five minutes, remains the type's most widely recognized example, and single-leaf and double-leaf bascule designs are still built today wherever a waterway must stay open to tall vessels.