A Rapid Rectilinear lens, or RR lens, is a symmetrical pair of cemented achromatic doublets. It was introduced by Dallmeyer in 1866. At the same time Steinheil invented a similar construction, the Aplanat lens. The outer lens elements of the two lens groups are concave meniscus lens elements, the inner lens elements are convex meniscus lens elements, made of another sort of glass. This lens construction reduces effectively radial distortion. Dallmeyer's original Rectilinear wide angle lens's rear lens group is smaller in diameter than the front lens group. The Rapid Rectilinear and the Aplanat each have two equal-sized groups. Steinheil offered a landscape Aplanat, a portrait Aplanat and a wide angle Aplanat. Many makers have made lenses of this design.