Rulers are printed with two scales because different regions settled on different measuring systems.

The longer version

One edge of a ruler is divided into inches with halves, quarters and smaller fractions; the other is divided into millimetres and centimetres. Carrying both lets a single tool serve drawings, patterns and hardware specified in either system, and lets a user convert roughly by reading across rather than by doing arithmetic. On many rulers the two scales start from opposite ends.

The split reflects history: units of measure grew out of local practice, and only gradually did one decimal system spread. Countries changed at different times, so older plans, machine parts and patterns still carry the earlier units. Keeping both scales on a ruler is a practical way to live with that mixture.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.