Bathroom sink drains appear to be quite simple but believe it or not there is centuries of trial and error behind the design. Plumbers never found it difficult to drain water from your bathroom sink, mainly because any sloping downward drain would work fine. The problem that plumbers faced was keeping the smells of the sewer out of the house. Unfortunately the same pipes that carried the waste water to the sewer systems also brought the pungent and potentially dangerous sewer gas back up into the bathroom.
For centuries people tried to solve the problem but it wasn’t until the 1800’s when the s-trap was invented that the problem was almost solved. How the s-trap works is that when the low bend in the s-trap pipe is filled with water, it effectively acts as a plug, preventing the volatile sewer gases from seeping back into the house. Now this worked in theory but unfortunately the rushing water going down the drain would create a vacuum sucking the remaining water down the drain with it, effectively breaking the seal. So each time you used your sink, to re-establish the seal you had to commit to memory to pour a little bit of water down the drain to fill the trap, of course you would be quickly reminded if you forgot by the reeking scent.
Finally in 1874 a plumber figured out that you had to equalize the air pressure in the drain system. By adding another piped called a vent that ran upward to the roof, this created a kind of pressure relief system so that sewer gases wouldn’t stink up the house. Later on a more effective version of the vented s-trap called the p-trap was introduced which ran down from under the sink, formed a “u” shape and the attached to the main drain pipe on a upward sloping horizontal to effectively keep water in the “u” shaped seal. It was dutifully named the p-trap because it looked like a “p” lying on its side. There were many other slight variations to the p-trap but needless to say, the p-trap effectively solved the problem of bad odors coming back up your sink drain to pollute your bathroom.