A best fitted sheet has its four corners and sometimes two or four sides, fitted with elastic, to be used only as a bottom sheet. The sole purpose of a fitted sheet is to keep it from slipping off the mattress i.e. providing better stability while the bed is in use
The elastic allows the fitted sheet to hug the corners of your mattress so while you are naturally moving during slumber; the sheet does not detach itself from the mattress. The variations & sizes are exactly as per the Flat Sheets.