at least 1.25 in.
2 in.
Two inches
4 inch or 110mm NO less then 11/4" and normally 11/2" depending on the code your using Most installations are 11/4" INLET and 11/2" outlet used in conjunction with a reducing slip nut (11/4 -11/2) 4" is the MAIN House trap or in some cases a strorm trap depending on the area that is being drained
I suspect that you have a "p" trap in either a floor drain, or possible the laundry tub or a sink somewhere in the house. If you look under the kitchen sink, or a lavatory in the bathroom you can see a p-trap, it's shaped sort of like the curved part of the letter p. The purpose of the p-trap is to trap, or prevent, sewer gasses in the sewer lines from backing up into the house. By design, p-traps catch and hold some water, which blocks the pipe and stops the gas from coming up through the drain. IF you have a floor drain, or a lavatory drain, either of which don't get used enough to keep the water in the trap from evaporating away, leaving the drain pipe open, then when a lot of water is poured or flushed, pumped out of a washer, then that flowing water will push sewer gasses back up through the drain with the dry p-trap. What you are smelling is sewer gas.
The same as any other P trap except it should empty into a properly trapped and vented fixture
P-trap may be cracked, nuts are loose on p-trap, or check for leak above p-trap that may be dripping down on p-trap.
Who invented the p trap
An S trap is not allowed in a plumbing system. Only a P-Trap
Its called a P trap because if you stand the trap up and looked at it, its in the shape of the letter P
There SHOULD be water in the "P-Trap" below the sink. The trap keeps sewer gas from entering the house. It's called a "P-Trap" because of the shape, it's shaped like the letter "P" on it's side. If the water drains from the trap, it's the right level.
P trap
A J trap is normally tubular and a P trap is brass and does not swivel