Chopping and channeling is a form of customization in the "kustom kulture" and among hot rodders. Both procedures are often combined, but can be performed separately.
"Chopping the top" goes back to the early days of hot rodding and is an attempt to reduce the frontal profile of a car and increase its speed potential. To chop a roof, the pillars and windows are cut down, lowering the overall roofline. Some racers on the dry lakes chopped the tops of their cars so severely the windows were only a few inches tall. These were sometimes referred to as "mail slot" windows. This sort of bodywork is also popular on custom cars, kustoms, and leadsleds. Roof chopping also quickly became very popular with drag racers for much the same reasons as it did for lakes racers. The first roof chopper is considered to be Sam Barris (brother of auto customizer George Barris), who chopped and customized his brand new 1949 Mercury. Barris also pioneered a more advanced form, where the B-pillar is removed turning it into a pillarless hardtop in the process.
Concept cars, such as the AMX-GT, often undergo a lowering of their roofs even if the vehicles are based on production models. Automakers use the chopping technique to make their show cars look more sleek and "racy", although it would be impractical for normal use.
To channel a car the body is lowered over the frame by removing the floor and refastening it higher inside the body, causing the body to rest closer to the ground without altering the suspension. The overall effect is to give the car's body a more massive appearance. Each automobile would have its own engineering challenges as far as modifying the various components of the chassis. Local laws may prevent making the modifications too extreme, and safety would dictate some restraint. Channeling is also popular amongst hot rod, leadsled and minitruck enthusiasts, though the latter refer to it as a "body drop".
Sectioning removes a horizontal section from the body of a car, lowering the top half onto the bottom half, and welding the result back together. The purpose is to reduce the overall height of the bodywork of the car. Like a roof chop, it also has the advantage of reducing a car's frontal area and reducing wind resistance. This sort of bodywork is popular on minitrucks, race cars, kustoms, and leadsleds.