I tell my girlfriend (d) about the formats of drawing sheets, we build a logical chain:

I: what do you call Watman, is the format A1, what then will be A2?
Half of A1.
I: Okay, what will be the A3 then?
Half of the A1.
I: again good, and then what will be the A4?
A: I am very happy – a booklet!