A boilersuit, or coverall , is a one-piece garment with full-length sleeves and legs like a jumpsuit, but usually less tight-fitting. Its main feature is that it has no gap between jacket and trousers or between lapels, and no loose jacket tails. It often has a long thin pocket down the outside of the right thigh to hold long tools. It usually has a front fastening extending the whole length of the front of the body up to the throat, with no lapels. It may be fastened with buttons, a zip, velcro, or snap fasteners . Boilersuits with an attached hood are available.
Boilersuits are so called because they were first worn by men maintaining coal-fired boilers. To check for steam leaks or to clean accumulated soot from inside the firebox of a steam locomotive, someone had to climb inside, through the firehole (where the coal is shovelled in). A one-piece suit avoids the potential problem of loosened soot entering the lower half of one's clothing through the gap in the middle. As the firehole opening is only just large enough for a fit individual to negotiate, a one-piece suit also avoids the problem of the waistband snagging on the firehole as one bends to wriggle through, or of jacket tails snagging if one has to come out backwards.
Coveralls are most often worn as protective clothing over "street" clothes at work, but sometimes instead of ordinary jacket and trousers.