World War I bomb sight designs had improved rapidly, with the ultimate development being the Course Setting Bomb Sight, or CSBS. This was essentially a large mechanical calculator that directly represented the wind triangle using three long pieces of metal in a triangular arrangement. The hypotenuse of the triangle was the line the aircraft needed to fly along in order to arrive over the target in the presence of wind, which, before the CSBS, was an intractable problem. Almost all air forces adopted some variation of the CSBS as their standard inter-war bomb sight, including the U.S. Navy and U.S. Army, who used a version designed by Georges Estoppey, the D-series.