[old_tv_nut]

Basic geometry. It's called a mask because it masks/hides at least two thirds of the phosphor dots (two out of three) from any one of the electron beams. For three beams and three phosphor colors, the maximum unmasked area is 33%, otherwise electrons from the wrong gun would spill onto the wrong color phosphor. Add some guard band for beam landing errors (purity adjustment), and you end up with about 20% of the mask area being open.