Jeff Linder wrote:

We must replace a 70-year-old toilet. In order to choose a toilet we are
told that we need to know the rough-in. What is this? On plumbing supply
web sites, such as kohler.com and americanstandard.com, there is mention of
12-inch rough-in. How do we determine our rough-in? And can we buy a
toilet without knowing that? Thank you. -Jeff Linder )

Rough-in dimension is the distance from the finished wall surface to
the center line of the waste outlet (pipe).
You can easily determine this by measuring from the wall (not the
baseboard) to the 2 bolts holding the bowl to the floor.
Note: very old bowls may have 4 bolts; disregard the 2 furthest from the
wall.

A 70 year old toilet could be 12" , or 10", or even 14".

10" and 14" RI toilets are still available (add roughly $100).

Once the old bowl is removed a very careful inspection of the
flange in the floor needs to be made. Often a 70 yr old one
is cracked or poorly connected to the closet bend.

Jim