manufacture the best ones out there,
heaviest elastic in the industry,
and using really good suspender clips.
Lengths are adjustable too.
Minis go across the corners,
while the big ones go under the mattress and come up int he corners to fasten to the sheet.
Big ones work best for sheets that are too big for the bed.
Both work for regular fitting sheets.
Also you can use non fitted sheets as fitted sheets since they are suspender clips and can hold the layers of fabric,
as opposed to the cheap ones at regular bedding stores.