Make the shift
A sheet of A4 halved is A5, halved again is A6, and the proportion never changes. It is the only paper standard that survives its own folding, and it makes a good grid for screens for the same reason it makes good paper.
A-Series is that idea written down: a set of card sizes, a type scale measured against the card rather than the viewport, and rules for when a card may grow beyond its ratio rather than clip what is inside it.
It is used here, on this site, which is the shortest documentation we could write for it.