An A4 sheet of paper

Aspect Ratio and Scale Factor

Fold an A4 along its long side and you are left with an A5: half the area, yet to the eye exactly the same rectangle. This is not a habit or a committee's taste. The sheet's side ratio is forced to be one specific number; no other number survives that fold. The 71% button on a photocopier is the same number wearing a different face.