More about The AFormat
& Silver Rectangle  page 1.

If we have a square, each of the sides of which measures 1 unit in length, the diagonal of that square measures ]/2. In Fig. 376 we see the said square. The diagonal is AB. The diagonal is laid out along the base line to AC, permitting us to construct rectangle ACDE. The long side is AC, the short side is AE. This rectangle has thus the proportions of the Aformat. But we do not see the sacred cut until we enlarge the diagram by constructing the small square's doublesize version.

We see this in Fig. 377 in which the arc CB is extended to point F, after which the doublesize version can be entered. We see that line ED is the upper horizontal sacred cut in square FGCA. The Aformat is not one particular size. It is a ratio, represented by five basic sizes, each twice (or half) the size of its neighbour. 