Fitted sheets are sheets with elastic at the mitered corners, allowing them to fit over the mattress and eliminate balled up, untucked and wrinkled sheets. While an ingenious invention, telling the top of the sheet from the sides is a tricky task. Determining which way a fitted sheet goes on the mattress is usually a method of trial and error; however, there are a few clues to look for to make the job a little easier.
Check the sheet's pattern. Provided the sheet is patterned with a floral or geometric design, this is the simplest way to put the sheet on right the first time. The top of the pattern will indicate where the top of the bed should be. Plain sheets, stripes or plaids are not helped by this method.
Lay the fitted sheet on the bed without securing the corners.
Eye or measure the length of two adjoining sides. With some sheets, the difference between the short and long sides is apparent, but with larger mattresses, such as king-size, a tape measure comes in handy. The top of the fitted sheet measures or appears shorter than the sides.
You may always start by putting the sheet on the bed; if it does not fit, turn it a half turn on the bed and it will fit.
Once you have found the top of the fitted sheet, mark it with a bit of colored thread to make it easier to find in the future.