...but were afraid to ask. This kind of stuff pleases me more than I can possibly say. And some of it is even useful:
If you prepare a letter, you will have to know the weight of the content in order to determine the postal fee. This can be very conveniently calculated with the ISO A series paper sizes. Usual typewriter and laser printer paper weighs 80 g/m^2^. An A0 page has an area of 1 m^2^, and the next smaller A series page has half of this area. Therefore the A4 format has an area of 1/16 m^2^ and weighs with the common paper quality 5 g per page. If we estimate 20 g for a C4 envelope (including some safety margin), then you will be able to put 16 A4 pages into a letter before you reach the 100 g limit for the next higher postal fee.
Isn't that handy? And my day was made by an offhand remark, a couple of paragraphs down, that German toilet paper is often A6 size.