Other plumbing fixtures have the drain trap in an out-of-the-way location, such as the kitchen or bathroom sink, where the trap is usually hidden in a cabinet under the sink.
When you look at the sink you cannot see the standing water, but if you follow the drain lines, you can see the required U or S shape where the water stands to trap the sewer gas.
Sink traps have the added benefit of trapping small objects dropped into the drain, plus they are fairly easy to remove.
Traps also collect hair, sand and other detritus and which also limits the size of objects that pass through into the rest of the plumbing.
You can take apart most traps for cleaning or they may have their own cleanout feature.
Large plumbing fixtures such as showers, tubs and washing machine drains also have drain traps, but they are not as easy to see because they are under floor level or behind walls.
A drain trap may seem like a strange name for a plumbing part unless you know its function.
A plumbing trap is designed to constantly hold some water, and that water keeps the sewer gas smells from escaping the drain.
There are traps in every drain because any connection that leads to the drain system is also a possible outlet for sewer gas.
When someone reports a strange odor in any room where there is a drain, the first thing I do is check is to make sure that the trap is not dry.
If a drain trap is dry, the sewer gas can escape and cause of the smell.
This is usually a quick fix that can be remedied by running water down the drain and filling the trap back up with water.
The location of the drain trap is often obvious; it may be in the toilet where you can always see the standing water in the bowl.
If you look to the backside of the toilet, you can see the shape of the drain through which the water exits, which has the necessary curve.