"The cornerstone (or foundation stone) concept is derived from the first stone set in the construction of a masonry foundation, important since all other stones will be set in reference to this stone, thus determining the position of the entire structure." Translated into baseball terms, I believe that's an everyday play, possibly an infielder [where every play begins in front of him], not a starting pitcher [directly involved every 5th day], not a relief pitcher [randomly called into a game], not a DH [never on the field] and not an outfielder [where the play has to come to them]. It's a play that produces everyday, offense or defense. In other words, he is capable in all of the "5-tools" of baseball. In other words, a guy other team members look up to; a guy you can build a team around.