> > One black line and one white line is a "PAIR" > of lines to me. Why would you need 4 lines > to make up a pair?
You don't. It's line *pairs* per mm if you count both the black and the white line. If you only count the black lines (treating the white as just the background) you get lines per mm. In other words: 100 line *pairs* per mmm is 100 black lines and 100 white lines. 100 lines per mm is 100 black lines (on a white background), or alternatively 100 white lines (on a black background). They all describe the same resolution pattern. The terminology using line pairs is technically more correct, but the other usage is widespread and is a closer analogue to the measurements used in diffraction gratings, which generally talk about rulings per mm.
