The 1972 International Regulations for Preventing Collisions at Sea and the United States Inland Navigation Rules provide for an elaborate series of day-shapes and lights to be displayed by large vessels whose maneuverability is restricted by the conduct of underwater operations, such as cable-laying, dredging, or conducting diving operations. Smaller vessels that are not able to hoist the complex signals used by large ships are instead required to display a rigid version of the International Code of Signals flag for the letter "A," known as ALFA, at least one meter high if diving operations restrict their ability to maneuver. Not all boats from which divers are swimming are necessarily so restricted. Generally, only vessels to which the divers are physically connected by communication lines, air hoses, or the like are affected by this requirement. It does not apply to most instances of sport diving, where the divers are swimming free of the vessel. As recent Coast Guard Notices to Mariners emphasize: "The ALFA flag is a navigational signal intended to protect the vessel from collision."