The Unstated Sheet
Pick up a ream of copy paper in the United States and read the wrapper. It says 8.5 × 11 in, and it says 20 lb. Neither number describes the paper inside. An A4 ream has no such problem. It says 210 × 297 mm and 80 g/m². Both of those are properties of the sheet you are holding: its dimensions, and its mass divided by its area. Check them with a ruler and a kitchen scale and they come out right. ...