Is the paternal grandmother (i.g., wife of the grandfather) considered a mahram?

It is forbidden for a man to marry his father's or grandfather's wife—whether the father or grandfather is a close or distant relative, an heir or not, and whether the relationship is by blood or by nursing—regardless of whether the father had consummated the marriage with her or not. This is based on the Almighty's saying: (And do not marry those [women] whom your fathers married, except what has already occurred).