While most private toilet seats are a straightforward oval design, many public bathrooms in the US have a curious horseshoe shape with an open-ended front (along with an offputting large gap around the door). Bathroom connoisseurs may have noticed it's a common feature of airplane toilets too.

Oddly enough, horseshoe toilet seats in public toilets are outlined in industry-standard regulations from the International Association of Plumbing and Mechanical Officials (IAPMO). As per their Uniform Plumbing Code: “Seats, for public use, shall be of elongated type and either of the open front type or have an automatic seat cover dispenser.”

This isn’t written into law, but most public buildings in the US tend to follow this guidance. 

The reason behind the U-shaped seats of American toilets has plenty of theories. One idea is that they are more flexible to deal with the diversity of ass sizes they must support. Another suggests it makes it’s to reduce the amount of surface that’s subjected to pee splatter. 

Hygiene certainly does appear to play a role in this design choice, but it appears to have more to do with avoiding contact between the dirty toilet seat and a user’s hand and/or junk, according to Lynne Simnick, the senior vice president of code development at the IAPMO.

Speaking to Slate magazine in 2013, she explained that it “eliminates an area that could be contaminated with urine” and “eliminates the user’s genital contact with the seat.”

She also suggested it was designed with women in mind, explaining it was designed to easily allow women “to wipe the perineal area after using the water closet”.

Of course, pee-wiping professionals among you will be quick to point out that people with a vulva should avoid wiping from back to front as this will significantly raise your risk of picking up a urinary tract infection (UTI). 

However, it does look like the mysterious shape seat can allow users to reduce contact with the potentially germy surface if, for example, they want to put some toilet paper in the bowl before they get down to business.