The tread is the outside portion of the casing of the tire, the part of the tire that comes in contact with the road surface. “Slick” tires don’t have treads – that’s why they’re slick – but all other tires do. If the tire has lines with a teeny bit of tread, we call that “inverted.” And if the tread pattern starts to raise itself from the innermost portion of the tire, we call that “exterior” tread. Knobby tread is pretty much what it sounds like; it includes knobs or lugs that stand alone, not bridging themselves to other protrusions, and not appearing in a pattern.