Why Some People Put Top Sheets On Upside Down read is in the first comment

over the mattress. Science tip: If your fitted sheet keeps popping

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off the corners, buy a cheap pack of “bed suspenders” (elastic straps with clips) that go under the mattress. It keeps the sheet drum-tight, which is wonderful for supporting your back while you sleep!
Step 2: The “Upside-Down” Placement
Take your flat sheet and lay it over the bed upside down. This means the decorative hem is at the top (near the headboard/pillows), and the rough seam/tag is at the bottom (near the foot of the bed). Make sure the sheet is centered and hangs evenly