There are different ways to define a franchise cornerstone, although it's easy to identify the NFL players who have earned that designation.
Some do so by playing their position at the highest possible level over a number of years. Others help their teams overcome front office and coaching errors, elevating mediocre rosters single-handedly.
If a player is the deciding factor in multiple great franchise moments through the years, that's yet another way to become a cornerstone.
Some stars fall into that designation even more than others. For example, one player could do more with less than another throughout his career, both from a coaching and a personnel perspective.