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In 1917, W. G. Blauvelt of AT&T developed the combination pattern of letters assigned to each digit, which became the standard in North America. Large cities like New York City would ultimately require a seven-digit telephone number. Some tests in the early 1900s indicated that the short-term memory span of many people was insufficient for seven digits, causing dialing errors (the documentation for these tests is lost).[citation needed] As large cities had both manual and automatic exchanges for many years, the numbers for manual or automatic exchanges used the same format, which could be either spoken or dialed.[1]