Before the Industrial Revolution, most things were produced using hand production methods.
As machine methods increasingly replaced hand production methods, standardisation across industries became highly important (imagine differently shaped USB ports).
So, Standards Organisations were set up across countries — their job was to create uniformity in product specifications, terminology, protocols, etc. across producers, consumers, and traders.
When international trade boomed, the individual standards of the countries proved insufficient and became barriers to trade.
So, International Standards Organisation (ISO) was set up in 1946 (Headquarters: Geneva, Switzerland; Member countries: 165).
Now, ISO has set more than twenty thousand standards across technology, agriculture, food safety, healthcare, etc.
One of the ISO standards, ISO 216, is for paper sizes — used across the world except in North America (uses Letter and Legal sizes) and parts of Latin America.
ISO 216 defines 3 series of paper sizes — “A”, “B” & “C”.
All the paper sizes (except some envelopes) in these series have an aspect ratio of √2:1, i.e. 1.41:1 (length:width).
This means the long-side will always be 1.41 times the size of the short-side.
The advantage of basing a paper size on √2:1 is the unique property it gives to the paper —when you cut or fold it in half down the long side, you end up with two equal new pieces of paper with the same aspect ratio √2:1.
So, if you cut A3 into half, you will have 2 A4 sheets with the same aspect ratio and if you cut A4 into half, you will have 2 A5 sheets.
Now, it started with A0, which is exactly 1 square metre in size (1m2, i.e 841mm x 1189mm).
A1 was half of A0, A2 half of A1, A3 half of A2 and so A4 ended up being 210 mm X 297 mm (and if you divide 297 by 210, you will get 1.41).
