What is the most commonly used paper size (210 × 297 mm) called?

Had to know that, spent 50 years in commercial printing

Harvey Katz, 8.5x11 it's "Letter" size defined by American National Standard Institute. It's close to A4 but not exactly the same - 215.9 x 297.4mm versus 210 x 297.

8.5 x 11 inches in the USA.

An A0 sheet is one square metre . Each following number halves the long edge . I don’t know the A0 size but A3 is 420 c 297 mm, so A4 becomes 210 x 297 , A5 is 148 x 210, A6 is 105 x 148 and so on

I was only familiar with blueprint sizes, A through E. But instead of A0 = A and getting bigger, A0 = E and gets smaller such that A4 = A.