The confusing bit is that A4 is not the same size as US Letter, but A4 is commonly used for letters alright...This is particularly annoying because Microsoft software tends to default to US Letter size, which at first glance looks similar to A4, but turns out to be distinctly dissimilar once you try to print your US Letter sized document onto A4 paper or vice versa.
A4 = 8.25" x 11.75" or 210 x 297mm
US Letter = 8.5" x 11" or 215.9 x 279.4mm
The nice thing about the A-system is that the short side dimension of a sheet is the long side dimension of the next smaller size sheet. You can traverse your way throughout the A-series range by starting with an A0 sheet and cut it in half to get A1, cut that in half to get A2, etc. So just like the 'copy series'
@snusmumriken discussed earlier.