Shimano in 1990 introduced the Shimano Pedaling Dynamics (SPD) range of clipless pedals and matching shoes, designed so that the shoes could be used for walking. The shoes have a recess in the bottom of the sole for fitting the smaller cleats and therefore it does not protrude, while conventional clipless road pedals are designed for road cycling shoes which have smooth soles with large protruding cleats, which are awkward for walking. The SPD range, in addition to other off-road refinements, were designed to be used with treaded soles that more closely resemble rugged hiking boots. SPD pedals and shoes soon established themselves as the market standard in this sector, although many other manufacturers have developed alternatives which may be less prone to being clogged by mud or easier to adjust. However, the SPD dominance in this sector has meant that alternative pedal manufacturers nearly always design their pedals to be usable with Shimano shoes, and likewise mountain bike shoe manufacturers make their shoes "Shimano SPD" compatible. SPD has spawned 2 types of road cleats which are incompatible with standard SPD pedals and some shoes – SPD-R and SPD-SL. SPD-R is a now defunct pedal standard. SPD-SL is basically a copy of the standard Look clipless pedal system. It has a wide, one-sided platform and a triangular cleat that is Look 3-bolt compatible.