Slipstreams are not symmetrical and they all go one way. I will call this left handed because it means that on the left hand side of the car they pass through the air from narrow edge to wide edge. On the right hand side they pass from wide edge to narrow, seemingly not so slippery to the air as on the other side. Wish I could get a set of right-handed ones. Organic chemists would designate these as the S-stereoisomer. The study of chemical chirality has ruined the aesthetic.