This listing is for 20 yards of thin white 1/4" rubber elastic.   Super elasticity.  This is the type of elastic used in commercial swimwear (like a rubber band)  Not for masks. Should be sewn into a casing between layers of fabric.  Do not top stitch or attempt to thread through a casing with a safety pin - in either case the elastic will be punctured and will break 

Your choice of:

extremely thin lightweight pictured on left in 2nd photo (.010" thickness)

Lightweight (.015" thickness)

or

medium weight pictured on right in 2nd photo (.020" thickness)

No returns on rubber elastic.