Hercules Graphics Controller
The first two video card standards released for the PC were MDA and CGA. MDA was a text-only display, it didn’t allow graphics, while CGA allowed graphics with a maximum resolution of 640x200. Each video card required a compatible video monitor, i.e. a MDA monitor didn’t work with a CGA monitor and vice-versa.
The video card, not by the video monitor, set MDA graphics limitation.
A company called Hercules created a video card called HGC that could use MDA video monitors to display graphics. Its resolution was 720x350, greater than CGA’s, but with just two colors (black and white, since color MDA monitors didn’t exist as they were originally projected for a text-only environment).
One kind of video card that was very common at the time was called ”dual“, which was at the same time MDA/HGC and CGA. When set to operate as MDA, these cards worked under Hercules mode, running at 720x350. Since MDA and CGA video monitors are different from each other, the mode you could run on your dual video card depended on the video monitor you had.
Figure 1: A "dual" video card.