The pennant numbers (a kind of registration number, also known as hull numbers) of US Navy vessels carry a prefix of 2-4 letters designating the class of the ship; this practice dates back to the closing decades of the 19th century. These codes are fairly frequently used and recognised as a shorthand for that ship class (of any nationality). Those of more than 3 letters are usually a general class with a couple of extra letters to indicate weaponry (-G = guided missile-armed) or propulsion (-N = nuclear-powered) The following list is not yet exhaustive. Some classes are obsolete; many vessels have been reclassified as they were withdrawn to supporting roles in the later stages of their working lives. The numeric portion (only) of the pennant number is shown on the ship's side in peacetime colour schemes. You're expected to be able to work out what sort it is by looking at it ...