There is a style of pillow case that works rather like a fold-top sandwich bag.
One hems the ends of a strip of fabric as wide as the desired case (plus seam allowances) and a bit more than twice as long.
Then fold the ends in, right side together, overlapping with the short end underneath the longer one so that it will be on top when the case is turned right side out. Sew the side seams.
One puts the pillow in by turning the short end inside out, putting the pillow in, then turning the short end right-side out over the pillow.
A sham would have the opening running lengthwise down the middle, and would probably be made of three pieces to allow a wide ruffle to go all around. (I've often wondered why they are called "shams" when there is a quite genuine pillow inside.)
My question is, what is this kind of opening called? It could be used on many sorts of cases.