Water closets are either S trap or P trap. These two features trap a small amount of water to prevent the backflow of sewage gases into the water cosset and finally back to the room creating a bad odor and allowing bacterial into the room.
Traditionally, most toilets were the S type. Italian S trap water Closet has an S shaped plumb fitting just below the water closets. This plumb fitting traps water in the S shape after much of the water has gone down the drain. This water acts as a sealant and prevents flow of sewage gases back to the closet. Due to its S shape, this design creates a siphoning effect which sometimes drains all the water in the S trap. Thus S trap toilets are being replaced by P trap which does not have siphoning effects.

Water closets are either S trap or P trap. These two features trap a small amount of water to prevent the backflow of sewage gases into the water cosset and finally back to the room creating a bad odor and allowing bacterial into the room.
Traditionally, most toilets were the S type. Italian S trap water Closet has an S shaped plumb fitting just below the water closets. This plumb fitting traps water in the S shape after much of the water has gone down the drain. This water acts as a sealant and prevents flow of sewage gases back to the closet. Due to its S shape, this design creates a siphoning effect which sometimes drains all the water in the S trap. Thus S trap toilets are being replaced by P trap which does not have siphoning effects.