The pedal wrench you use to put the pedals on and off is 15mm,
but that is not the tread size.

Older French stuff would be using all metric rounded down numbers, Fuji , Japan, would use fractional parts
metric equivalents come out as decimal portions of a MM 25.4=1", 22.2 =7/8" for example .
one piece cranks on cruiser bikes will have 1/2" pedals