A custom car is a passenger vehicle that has been modified in either of the following two ways. First, a custom car may be altered to improve its performance, often by altering or replacing the engine and transmission. Second, a custom car may be a personal "styling" statement by the re-styler/re-builder, making the car look "unique" and unlike any car that might have been factory finished. Customs are distinct from hot rods; exactly where the difference lies has been the subject of debate among customizers and rodders for decades.
With the change in automobile design to encase the wheels in fenders and to extend the hood to the full width of the car, the former practices were no longer possible. In addition, there was tremendous automotive advertising and subsequent public interest in the new models in the 1950s. Hence custom cars came into existence, swapping headlamp rings, grilles, bumpers, chrome side strips, and tail lights, as well as "frenching" and "tunnelling" head- and taillights. The bodies of the cars were changed by cutting through the sheet metal, removing bits to make the car lower, welding it back together, and adding a lot of lead to make the resulting form smooth (hence the term "lead sled"; lead has been replaced by Bondo). By this means, "chopping" made the roof lower; "sectioning" made the body thinner from top to bottom. "Channeling" was cutting notches in the floorpan where the body touches the frame to lower the whole body. Fins were often added from other cars, or made up from sheet steel. In the custom car culture, someone who merely changed the appearance without improving the performance substantially was looked down on.
Paint was an important concern. Once bodywork was done, the cars were painted unusual colors. Transparent but wildly-colored candy-apple paint, applied atop a metallic undercoat, and metalflake paint, with aluminum glitter within candy-apple paint, appeared in the 1960s. These took many coats to produce a brilliant effect — which in hot climates had a tendency to flake off. Customizers also continued the habit of adding decorative paint after the main coat was finished, of flames extending rearward from the front wheels, scallops, and hand-painted pinstripes of a contrasting color. The base color, most often a single coat, would be expected to be of a simpler paint. Flame jobs later spread to the hood, encompassing the entire front end, and have progressed from traditional reds and yellows to blues and greens and body-color "ghost" flames.
Once customizing post-war cars caught on, some of the practices were extended to pre-war cars, which would have been called fendered rods, with more body work done on them. An alternate rule for disambiguation developed: hot rods had the engine behind the front suspension, while customs had the engine over the front suspension. The clearest example of this is Fords prior to 1949 had Henry Ford's old transverse front suspension, while 1949 models had a more modern suspension with the engine moved forward. However, an American Museum has what could be the first true custom, built in 1932, amongst its exhibits.
With the coming of the muscle car, and further to the high-performance luxury car, customization declined. One place where it persisted was the U.S. Southwest, where lowriders were built similar in concept to the earlier customs, but of post-1950s cars.
Others became notable for their appearances in film (such as Ala Kart, The California Kid five-window, or the yellow deuce from "American Graffiti") or television (such as The Monkeemobile, the "Munsters" hearse, or, more recently, Boyd's full-custom "Tool Time" '34, or Pete and Jake's '33 three-window, Eliminator, built for the ZZ Top video). Specialist vehicles, such as the T/A, KITT, from "Knight Rider", are not usually considered customs, but movie or TV cars, because they retain a mostly stock exterior.