Several systematic telephone numbering plans existed in various communities, typically evolving over time as the subscriber base outgrew older numbering schemes. A widely used numbering plan was a system of using two letters from the central office name with four or five digits, which was designated as "2L-4N" or "2L-5N", or simply "2–4" and "2–5", respectively, but some large cities initially selected plans with three letters ("3L-4N"). In 1917, W. G. Blauvelt of AT&T proposed a mapping system that displayed three letters each with the digits 2 through 9 on the dial.