I recently went down the A4 paper sizes rabbit hole and found out that there is an ISO standard for this 'ISO 216'

https://en.wikipedia.org/wiki/International_standard_paper_s...

Edit: while going through the article again, I noticed that's a brief mention of the ISO standard, which I missed in the first round.

Some Moleskine cahier notebooks are wrapped in a paper sleeve that has a ruler printed on the back. Inches on one edge; centimetres on the other (a slight improvement for non-American users over Field Notes notebooks, with a ruler on the inside back cover that’s inches only - also the sleeve is rather longer, 28cm in fact).

21cm is close to 20 which means A5 has a side close to 10.

29.7 is close to 30. So why not use A4 sheets to install kitchen cabinets? A friend of mine advised me to buy a laser level. A tiny level was quite enough. A laser meter is nice though although I don't understand why it's inaccurate sometimes. Maybe it depends on the surface matter/paint.

I have heard German toilet paper is A6, but I never got to verify that. If true, it's one of the more German things ever.

A0 paper is for drafting. That's what people used for engineering drawings before we had AutoCAD with zoom.

Sqrt(2) being halfway between 1 and 2 multiplicatively, leads to interesting stuff. For example consider two integers A and B. They have a dual in the pair A+B and A-B. Well, not quite. You need to scale them down by 1/sqrt(2). If you do that to the duals, you get the originals again.

As the ratio used is a rational approximation of an irrational, I would guess that the ratio-preserving feature breaks down well before you get to atomic sizes, though I have neither proved that to be so nor figured out how to calculate the divergence.

If you have a Pro or Pro Max model of iPhone from the last several years, it has a LiDAR that allows the pre-installed Measure app to measure lengths/heights, etc., using the camera. Several higher end Android phones may also have the same.

Adds gsm issue to https://github.com/librasteve/raku-Physics-Unit/issues/32