ISO 216
Not to be confused with ISO 16, about the pitch A4.
ISO 216 specifies the size of the A, B, and C series of paper, most famously A4 and A5.
Specifically, A4 paper is 210 mm × 297 mm, while A5 paper is 148 mm × 210 mm.
As the aspect ratio of ISO 216 paper is always √2:1, a two-page A5 spread is the same size as a single A4 page on its side, and so on.