Well, are you going to tell the story of how they came to exist during WWI or make me research it out myself?
sigh
During WWI there was plenty of .45acp ammo to go around, but a distinct shortage of Browning's 1911.
Department of War officials approached Colt and S&W, to urge them to make 1911s for the war effort. Colt and S&W explained that the capacity to make revolvers did not roll over into an ability to make semi-automatic pistols.
An ingenious gun smith (who's name I refuse to look up), came up with the idea of half moonclips which held 3 rounds of .45 each. Two half moon clips would reload a .45 revolver.
The moonclips are necesary since the .45 acp round is rimless. With no rims for the revolver extractor to push against, it would be difficult to get the empties out and reload the revolver. The clips allow purchase for the extractor to do its job and as a bonus, the clips speed the process of loading fresh rounds.
The full moon clip which held 6 rounds was a later innovation after the war. I've read somewhere that with the half moon clips, the rounds would fit in the ammo pouchs of the time.
With practice, an accomplished pistoleer could reload the moonclip revolver as fast or faster than an automatic magazine.