In old five-story buildings, you can find small windows between the kitchen and the bathroom.
This was reported by URA-Inform with reference to Рixelinform.
Although it is not known exactly what they were intended for, there are several theories explaining their functions. The most logical explanation is energy saving. The light from the kitchen allowed the bathroom to be illuminated, so it was possible to do without a separate light source in the toilet.
Another theory suggests that the windows helped to expand the space of small rooms, adding the illusion of greater volume.
There are also more specific assumptions. One of them is emergency assistance: if a person locked himself in the bathroom and needed urgent help, the window could be broken to get to him.
Another version is related to protection from a blast wave in the event of a gas accident. Since such houses often had gas water heaters and boilers, the window served to transfer the pressure to it, while maintaining the integrity of the wall.
There is also a version about sanitary purposes. In the closed, damp space of the bathroom, dangerous bacteria could multiply, including the tuberculosis pathogen, and the window provided access to ultraviolet light, which reduced the risk of infection.
Finally, the window could provide natural ventilation of the bathroom, preventing the accumulation of moisture and the appearance of mold, which reduced the need for constant ventilation through an open door into a narrow corridor.
Recall that it was previously reported why drivers should eat a slice of lemon before long trips: the answer will surprise many.