Because large ships are complicated internally, and may take a long time to construct, as much as five to 10 years, it is rare to have two of them that are completely identical. It may also be that the second and later ships must be started before the first one is even launched. Nevertheless, it is still more efficient to build copies than prototypes, and so the lead ship will be the one that guides the construction of the others in its class.
From the 20th century to the present, the lead ship usually loans its name to the class, as in Pennsylvania class battleship, whose lead ship was the USS Pennsylvania (BB-38), or more recently Los Angeles class submarine named after the USS Los Angeles (SSN 688). In the latter case, the Los Angeles's hull number "688" is also used informally to refer to the class. The Royal Navy occasionally gives all ships of a class names that have something in common -- perhaps simply all beginning with the same letter -- so that class is known not only by the name of its lead ship, but also by whatever their names have in common. Thus, HMS Zulu was a Tribal-class destroyer even though there was no ship named HMS Tribal, and the Amphion class of submarines are also known as the "A" class.