Complex sentence: Outside of North America, Letter size may also be known as "American Quarto" and the size is indeed almost exactly one quarter of the old Imperial (British) paper size known as "demy quarto" (× in), allowing inch for trimming.
Split into simpler sentences: Outside of North America, Letter size may also be known as "American Quarto". The size is indeed one quarter of the old Imperial paper size known as Demy (in × in), if one accepts a quarter to half an inch for trimming.