digits 2 to 9 in the middle position, to eliminate conflicts with the area codes. 1951 to 1960 Direct Distance Dialing (DDD) trials were conducted by the end of 1951, in which subscribers in Englewood, New Jersey, could dial long-distance calls to a select group of remote destinations, as far away as San Francisco, by using an area code. DDD technology expanded to other major cities, By 1960, it was available in a few places in Canada, as well as most large American cities. This decade is notable for implementation of some thirty more area codes, including in Alaska, Hawaii, and the Caribbean. 1960 to 1981 Major progress may be noted in provisioning DDD service. Long-distance billing was computerized in the 1960s to the early 1980s. Few area codes were introduced during this time. Toll-free 800-service was introduced. Service demands in the largest American cities of New York and greater Los Angeles area resulted in the first use of interchangeable central office codes (NXX). All-number calling was implemented, replacing 2L-5N telephone numbers. Although a very small number of places still had did not use a seven-digit numbering plan by 1981, they were rare. 1981 to 1994 New area code assignments were increasing in the 1980s. From 1990 to 1994, all remaining assignable codes entered service. The exhaustion of NXX codes necessitated interchangeable codes in several more area codes. Ten-digit dialing, or 1+10D, was implemented in area codes with interchangeable NXX codes. Protected dialing plans as in national capital areas were discontinued to help meet demand without area code relief. As 1994 neared its end, ten-digit dialing became required throughout the numbering plan in preparation for interchangeable NPA codes. All local numbers now had seven-digit, as the last technological hold-outs had given way to modern switching technology. The concept of a ten-digit local number was conceived, as New York had an overlay code (917, implemented in 1992), but seven-digit dialing was still the norm. 1995 to 2019 Interchangeable NPA codes were introduced in Washington and Alabama, and some forty new area codes were introduced in 1999 as relief was implemented for pent-up demand. This included two additional toll-free prefixes as the 888 code was quickly exhausted. Since 1999, a more steady rate of area code introductions has taken place, the rate being slower due to one or more factors: economic recession consumer resistance conservation measures or regulatory measures pent-up demand being