I have a king bed but bought california king sheets for extra room.  I have a pillow top mattress plus a 4" memory foam mattress topper and all my sheets slip off at the corners.  It's been so irritating until now.  These sheets are no doubt the best ones I've ever purchased and are so soft.  There is enough room and elastic at the corners and they do not come off.  They extend to cover all my toppings and the elastic holds them there.  The material isn't very thick but that's OK for Florida sleeping and they seem to stay cool.