A flat two conductor, type SBT parallel cord, with an additional grounding conductor has a moulded ridge on the exterior surface of the insullation, running the full length of the conductor. Which conductor does this ridge identify?

A flat two conductor, type SBT parallel cord, with an additional grounding conductor has a moulded ridge on the exterior surface of the insullation, running the full length of the conductor. Which conductor does this ridge identify?