VERY EFFECTIVE 144

Bought for the first time, I use to tighten the flat sheets (1 single bed) of my daughters who are unable to stretch their sheets properly because the mattress returns of about 5cm under their bed
In the morning, the sheet does not come out from under the mattress, there is no need to lift the mattress to re-engage the sheet below. They have, so to speak their beds and to me either.
Note, "tension" are short and allow either stretch the width of flat sheets for beds 90, or for large beds, soft fitted sheets at their corners (diagonally) that they held at bed if the mattress is not very thick and fitted sheets are not resistant to movements / the "sleeper (s)" and removed.