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In the real world, the term cruiser originates in the age of sail, when it denoted simply a ship assigned to cruise independently of large battlefleets, either as a scout operating ahead of the main fleet, or on detached duty.
This of course defined cruisers as ships which did not fight in the line of battle, so that the term was particularly applied to frigates, fast, mid-sized vessels that were not designed to stand up to the firepower of ships of the line.
The inter-war naval treaties formally defined cruisers by three main constraints: limiting their displacement to 10,000 tons; redefining a "light cruiser" with nothing more than six-inch guns; and formalizing a new term, heavy cruiser, for ships with guns of up to eight-inch caliber.
Against the German pocket battleship (heavy cruiser) Graf Spee one heavy and two light British cruisers were able to split the fire of her heavier guns and although damaged, trail her to port where she was subsequently scuttled rather than risk battle again.
Cruisers were also attached to the main battlefleet and used for reconnaissance, sweeping ahead of the fleet looking for the enemy.
Light cruisers were defined to be armed with 6.1 in (155 mm) guns or smaller and heavy cruisers to be armed with larger calibers, 8 in (203 mm) being particularly common.