Six sides, and why the sixth one matters
A fitted protector covers five surfaces at best and in practice usually three. The underside is left open. That is where a bed bug harbors, and it is where liquid ends up once it has run off the side of the mattress.
An encasement zips around all six surfaces. It is more work to fit, once, and no work at all after that.
Waterproof, breathable, closed with the patented BugStop® Seal.