Looking under the sinks in many older homes it is not unusual to see the original trap on the drain, these were often similar to an “S” on it’s side.
They are no longer permitted due to their tendency, when draining water from the sink, to finish with a siphon action that can empty the trap of water which in turn allows sewer gasses (sewage odor) into the home. Sewer gasses which can have a noxious odor are also potentially full of germs and pose a health hazard.
The standard trap has the pipe down from the sink into the trap (the “U”) and the pipe from the trap goes straight back to the main drain, not down like the S-trap.
Surprisingly the hated “S-trap” is still sold, and DIY plumbers install them all the time unaware of the problems and health issues relating to them.