Main drain
The fitting, or pair of fittings, at the pool's deepest point that pulls water from the bottom into circulation alongside the skimmer.
The main drain sits at the lowest point of the pool and draws water from the bottom into the circulation system, working alongside the skimmer (which draws from the surface) so the pump pulls evenly from top and bottom. Modern pools use two main drains, spaced apart, specifically to prevent the suction entrapment hazard a single drain can create; this is required by federal law (the Virginia Graeme Baker Act) on public pools and is standard best practice residentially. Main drain covers should be inspected periodically for cracks or looseness, since a damaged cover is one of the few genuinely dangerous equipment failures a pool can have.
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