Toilet slippers

In Japanese culture, there is a tendency to separate areas into clean and unclean, and the contact between these areas is minimized.

For example, the inside of the house is considered a clean area, whereas the outside of the house is considered unclean. To keep the two areas separated, shoes are taken off before entering the house so that the unclean shoes do not touch the clean area inside of the house.

Historically, toilets were located outside of the house, and shoes were worn for a trip to the toilet. Nowadays, the toilet is almost always inside the home and hygienic conditions have improved significantly, but the toilet is still considered an unclean area, even though other places are much more likely to have higher bacterial contamination.^[44]

To minimize contact between the unclean toilet floor and the clean floor in the rest of the house, many private homes and also some public toilets have toilet slippers (トイレスリッパ toire surippa^?) in front of the toilet door that should be used when in the toilet and removed right after leaving the toilet.^[2]
This also indicates if the toilet is in use. 

A frequent faux pas of foreigners is to forget to take off the toilet slippers after a visit to the restroom, and then use these in the non-toilet areas, hence mixing the clean and unclean areas.^[45][46][47]