Etymology of the day: flat.
In a well known difference between US and UK English, the things that are called apartments in the US are here called flats. This turns out to be not because, as my young mind had many years ago imagined, flats are flat across one floor, unlike houses, which have an upstairs, but instead from an older middle English word flet, meaning a dwelling of any kind.
I found out about this because in Tolkien the elves of Lórien are said to live in/on flets in the trees, and Tolkien really knew his middle English.