Often, the assumption is erroneously made that the large numeral, found normally either on the top of the handle or on the bottom of a piece, indicates its diameter in inches. A measurement of both the top and the bottom rim of a pan, however, will quickly confirm that the number has nothing to do with either dimension.
With the advent of wood-burning stoves, pans were produced to conform to the sizes of the openings in their tops known as "stove eyes". Think of a stove eye as a burner on a modern stove. (Some people still call them that.) A heavy cover piece was left in place when an eye was not directly in use, and removed-- using a special, heat-resistant lifter handle-- when maximum heat from the eye was required.
Heat rings, the rims protruding from the bottom circumference of most early cast iron pans, served multiple functions: as a "seal" of sorts between the pan and stove eye, as added stablility for less-than-perfectly flat pan bottoms, and as a device to help reduce hot spots.
Depending on the brand of stove, and the sizes of its various eyes, appropriately-sized pans would need to be purchased for use with it. Or, in some cases, the stove maker also produced pans, which they supplied for use with their units.
So that's what that number is
