User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-4189499-20150520014724/@comment-4189499-20150520020552

All I know is that my screen is 2160 x 1440px, which puts it in the XL Desktop category, but it's actual size is about that of an A4 piece of paper, which puts it in the middle categories, and it's displaying at what I would expect to see from the middle category sizes.