Although the CSBS and similar designs allowed the calculation of the proper flight angle needed to correct for windage, they did so by looking downward out of the aircraft, so the correct angle was not easily visible to the pilot. In early bombers, the bomb aimer was normally positioned in front of the pilot and could indicate corrections using hand signals, but as aircraft grew larger it became common for the pilot and bomb aimer to be separated. This led to the introduction of the pilot direction indicator, or PDI. These consisted of a pair of electrical pointers mounted in a standard diameter instrument panel mount. The bombardier used switches to move the pointer on his unit to indicate the direction of the target, which was duplicated on the unit in front of the pilot so he could maneuver the aircraft to follow suit.