Plumbing – Where should this seal ring from a syphon go

plumbing

I'm from Europe and I do not know if you in the US have the same system concerning washing basin plumbing.
Today the water of our washing basin did not go away.
So I took my pipe wrench and opened the syphon in order to clean it.
Inside my syphon there is a plastic tube, I pulled it out and pulled all the mud out of the syphon with a tooth brush.
This was not the first time I did this. But this time I suddenly had a seal ring in my hand and I did not know where to put it.
Because I did not know (and still don't know) where to put the seal ring in the syphon I left it away and put all things together. But now the syphon leaks.
This is the only seal ring I recognized, it is a plastic, non-flat ring with a diameter of 4.5 centimeters.
Does anybody has an idea where to put this seal ring?

Best Answer

I'm not sure due to different terminology but I think you're referring what we call where I live a p-trap. If so, the washer is most likely the slip-joint washer as seen in this photo:

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There is one where the tailpiece connects the trap as well as one where the elbow connects to the trap's outlet. This second joint will always have to hold water as the trap remains filled past this point.

If this is not what you are referring to, a picture would help.