The Player’s Handbook (pg. 196) reads
Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.
However it only defines degrees of cover (1/2, 3/4, Full) in terms of area covered. RAW it then seems that a curtain of silk provides as much cover cover against the attacks of a battle axe wielding Orc as a stone wall. Reasonably, however, it would make more sense just to confer the benefits of Unseen rather than the benefits of cover.
Rules as Written is there something I am overlooking and if not what is there a more reasonable system of adjudicating cover?