A Grade Ring has grooves on the underside and a tongue on the upper side to form a tongue and groove joint between multiple Grade Rings, Grade FLAT or Finish Rings. A manhole frame should never be installed on a Grade Ring. The Grade FLAT Ring has only grooves on the underside and a flat top for installation of the manhole frame. Grade Flat Rings will not accept the Angle Ring. Finish Rings have grooves on the underside and are mostly flat on top with a groove located on the inside diameter which is there to accept and lock in an Angle Ring if used.