"The Divine Proportion, or Golden Ratio, is equal to (sqrt(5)+1)/2,
approximately 1.61803. It appears frequently in nature, the ancient Egyptians
probably knew about it, the ancient Greeks certainly knew about it, and the
artists and architects of renaissance Europe were obsessed with it.
Moleskine "pocket" notebooks are 9x14cm in size, only 4% from the Golden
Ratio (the long side is approximately 5mm short). A size of 8x13cm would be even
closer (within 0.5%) but would have 17% less area per page. 8.9×14.4cm would be
closer still (within 0.004%) but would not have the convenience of
whole-centimetre dimensions. 9x14cm therefore represents a balance between
useful area, pocketable size and convenient dimensions while remaining very
close in aspect ratio to the long-held ideal. The "large" notebooks, at 13x21cm,
are within 0.2%…"
The Divine Moleskine