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Deep pockets have recently been added to fitted sheets to provide for the added height of pillow-top mattresses. While these pockets are useful for larger mattresses, they can be too loose for standard mattress heights, coming undone no matter how you tuck them. Thankfully, you can reduce deep pockets as long as the elastic on the fitted sheet is only on the corners and not the entire circumference of the sheet.
Measure the length of the mattress to determine by how much you need to reduce the sheet.
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Cut the sheet down the middle from top to bottom and remove a strip of fabric equivalent to the required reduction.
Place the two sheet pieces wrong sides facing out and pin the ends together.
Sew the sheets back together making at least three passes with the sewing machine.