Three different ways to keep a trap seal from being siphoned away — and why they trade pipe count against layout precision instead of being interchangeable options.
Every plumbing fixture — a sink, a water closet, a floor drain — has a trap: a curved section of pipe that holds standing water. That standing water, the trap seal, is the only thing physically blocking sewer gas from traveling up the drain line and into the building. It's a remarkably simple defense, and it has exactly one weakness: it can be sucked out of the trap by the very water flowing past it. Venting exists to stop that from happening — and individual, common, and wet venting are three different, code-recognized ways of getting the same relief air to the same place.