Large ships are complicated internally and may take as much as five to ten years to construct. Any changes or advances that are available when building a ship are likely to be included, so it is rare to have two that are identical. Constructing one ship is also likely to reveal better ways of doing things and even errors.
The second and later ships are often started before the first one is completed, launched and tested. Nevertheless, building copies is still more efficient and cost-effective than building prototypes, and the lead ship will usually be followed by copies with some improvements rather than radically different versions. The improvements will sometimes be retrofitted to the lead ship. Occasionally, the lead ship will be launched and commissioned for shakedown testing before following ships are completed, making the lead ship a combination of template and prototype, rather than expending resources on a prototype that will never see actual use.
Ship classes are typically named in one of two ways; echoing the name of the lead ship, such as the Pennsylvania-class battleships, whose lead ship was the USS Pennsylvania (BB-38), or defining a theme by which vessels in the class are named, as in the Royal Navy's Tribal-class frigates, named after tribes of the world, such as HMS Mohawk (F125). If a ship class is produced for another fleet, the first active unit will become the lead ship for that fleet; for example, the Oliver Hazard Perry-class frigates are known as the Adelaide class in the Royal Australian Navy. Larger civilian craft, such as the Sun Princess, the lead ship of the Sun-class cruise ships, sometimes follow this convention as well.
The same custom is often followed in fiction: the Constitution-class cruiser is the basis for the Enterprise of Star Trek (although in Star Trek the term pathfinder is also occasionally used in lieu of lead ship) and the Imperial-class Star Destroyer appears in Star Wars.