While many guns can engage multiple types of targets, dual-purpose guns may be defined as those specifically intended to engage both surface and air. They generally appear in lieu of separate low-angle and high-angle batteries. By these criteria I suggest that the first dual-purpose armament appeared in the British aircraft carriers Glorious and Courageous, converted from "light battle cruisers" under the Washington Treaty. These carried sixteen single 4.7"/43 caliber guns, pairs on forecastle and quarterdeck and the rest in sponsons at main deck level. These were the same guns that appeared as AA armament in Nelson and Rodney, but sixteen guns were far in excess of AA requirements as understood at the time. The placement of most of the guns was also similar to that of capital ship secondary armament. Anti-surface armament was considered a necessary feature of carriers at the time. Comtemporaries like Furious and Eagle had 5.5" or 6" low-angle guns plus 4" AA weapons. Foreign carriers Hosho and Bearn had both types, while USS Langley had only low-angle guns. Larger Washington conversions like Akagi and Lexington featured 8" guns with 4.7/5" AA. So it seems likely that the 4.7s on G&C were intended to fulfill the low-angle role, at least to fend off destroyers or light cruisers, in addition to AA; as far I know, the first time a battery was specified to serve both purposes. One oddity, the identical guns on Nelson and Rodney would not be described as DP, since those ships had a dedicated 6" low-angle secondary.