A boom generally floats on the surface, while a chain can be on the surface or below the water. A chain could be made to float with rafts, logs, ships or other wood, making the chain a boom as well. Especially in medieval times, the end of a chain could be attached to a chain tower or boom tower. This allowed safe raising or lowering of the chain, as they were often heavily fortified. By raising or lowering a chain or boom, access could be selectively granted rather than simply rendering the stretch of water completely inaccessible. The raising and lowering could be accomplished by a windlass mechanism or a capstan.