bed sheet clips
Customer Service:

Drawstring sheets work great on futons

Futons mattresses are less ridged and are more irregular in shape than regular mattresses. For this reason, it is extra tough to find sheets that fit well. Since the Comfy-Snug drawstring sheet is self adjusting, it can match the contours of futon mattresses and grip it tightly.
bed sheet clips
Viewed for beneath the configuration of the hem and drawstring can be seen holding the sheet tight. For a futon couch this allows the mattress to be folded and unfolded without the need to replace the sheet.