Originally, transit lines in the region only used geographic names; though numbering was added to public maps in 1936. The three heavy-rail lines were assigned numbers 1, 2, and 3; what is now the Green Line was assigned different numbers for each branch. However, riders generally continued to use the geographic names. Colors were assigned on August 26, 1965 as part of a wider modernization under design standards developed by Cambridge Seven Associates, and have served as the primary identifier for the lines since then. The numbers for the heavy-rail lines and the Mattapan Line were retained in public information until 1966. In 1967, the then-current five branches of the Green Line were lettered A through E.