The middle column is changing by at most one number. Furthermore, it's changing by at most one number always in the same direction, a feature not shared by the

The middle column is changing by at most one number. Furthermore, it's changing by at most one number always in the same direction, a feature not shared by the right column. (The direction thing doesn't matter for binary grey codes, since 1 + 1 = 1 - 1 = 0 and 0 + 1 = 0 - 1 = 1.)