hospital corners 
plural noun
Overlapping folds used to tuck sheets neatly and securely under the mattress at the corners, in a manner typically used by nurses: I can do hospital corners...
- And, worst case, it can't hurt to learn how to fold proper hospital corners on a mattress with flat sheets.
- ‘It's like an apartment,’ said Lori, pulling tight hospital corners into her sheets.
- I like crawling naked into my own clean sheeted, perfectly made bed (I do hospital corners as well as smut) and having a nap still smelling like men's cologne.