After many disappointing purchases in the past looking for deep pocket sheets that would fit my 19" mattress, I thought that I finally found something that would fit properly. So the first thing I did after opening the package was get out my trusty ruler and measure the pocket. Sadly, the actual pocket ONLY measured a mere 15"! So I thought how in the world would these fit 21"? So I then saw that the pocket was not only gusseted on the top, but the bottom also. So this added the extra fabric to create 21" when measured (the gusset was 6" deep). So I put them on my mattress and it did fit. The top, bottom, and the corners stay in place but the sides don't. There isn't enough fabric to tuck under to keep them in. So far these are the closest fitting sheets I've been able to find under $100. I've used them for a week and nothing fell off except for the sides. This was after washing of coarse. Also the material is a little thinner than I expected and they do wrinkle somewhat. Just take them out of the dryer immediately when finished and don't over-dry. Hope this helps.