As Doc said: "In construction framing (at least here in the USA) standard
spacing of wall
studs, floor joists, as well as some other things is 16" on center. "

But never having thought about it  - only built many structures to this
requirement - it begs the question where did 16" come from? Why not 12"? I
understand that railroad tracks in 2014 are sized to Roman chariots of
likely 500 BC - so that's where that standard came from. But if we framed
to 12, 18, or 24 inch all engineered beams and calculations would have
easily accommodated - spans et al would simply be different and plywood
might be sized differently though 48 x 96 is still easily and completely
divisible by all but 18.

Completely useless question but I find myself intrigued nonetheless by the
answer. And it isn't just "Because"

Scott in Duluth