A few issues back, there was a thread about NN0 codes as central office codes.
AT&T's publication "Notes on Distance Dialing" (1975) [1] includes a list of 63 NN0 codes that could be assigned either as area codes or as central office codes. This list, identified as "Chart 5," includes all NN0 codes in the range 220-990 (except for 950 which "is reserved for a future network-wide service") in an arbitrary (non-numerical) order. Each code is identified by a "sequence" number (#1 - #63). A copy of the list is posted at
The accompanying text states:
2.03 Sometime after 1995, it is estimated that the 21 NPA codes still unassigned (end of 1974) will have been used and that it will be necessary to start using NNX type codes as NPA codes. In the interest of minimizing ambiguity, it is planned to assign the NN0 codes first in accordance with the sequence shown in Chart 5. (The NN0 codes have been designated as the last to be assigned as CO codes and a sequence that is the reverse of the of that for NPA code assignment is recommended.) Ultimately, it will become necessary to assign the remaining NNX codes for NPA code purposes. [2]Notes [3] on Chart 5 clarify the order of assignment:
Central office codes should be drawn from the list in sequence-number order.
After all N0/1X area codes are exhausted, further area codes should be drawn from the list in reverse-sequence-number order.
If I understand this correctly, the idea was to assign NN0 codes as central office codes *and* as area codes *before* the introduction of interchangeable area codes, but to draw from opposite ends of the NN0 list in order to prevent ambiguity. Presumably, this would have forestalled the need for interchangeable area codes until the list was exhausted, theoretically somewhere in the middle of the list.
It didn't work out that way ...
Many NN0 codes were assigned as central office codes whenever and wherever they were needed, without regard to their positions on Chart 5.
Examples that come to mind:
702-870 (#3 on the list) ca. 1989 Las Vegas 312-990 (#32 on the list) ca. 1988 Hinsdale 201-460 (#36 on the list) ca. 1982 Lyndhurst 414-730 (#52 on the list) ca. 1986 Appleton 214-680 (#54 on the list) ca. 1983 DallasI assume that one reason for selecting these combinations was an attempt to maintain the look and feel of existing central office codes. That was certainly the case in Appleton, where Wisconsin Telephone was already using several other 73X combinations as central office codes.
Curiously (as Mark Roberts noted in TD 24:482), 530 (#1 on the list) was in service -- at least briefly -- in California in 1965, a decade before Chart 5 was published. I wonder if this was just a coincidence? Or had some early version of Chart 5 already been published in 1965?
No NN0 area codes were assigned before 1/1/1995 (when interchangeable area codes were introduced), but once the floodgates were open, many NN0 codes appeared quickly. But they too were assigned as needed, without regard to their positions on Chart 5. Seven of them were assigned [4] during 1995:
360 (#6) Washington 630 (#15) Illinois 770 (#25) Georgia 540 (#29) Virginia 970 (#31) Colorado 860 (#34) Connecticut 520 (#61) ArizonaNote that Washington's 360 (#6 on the list) was actually #58 in the reverse sequence, while Arizona's 520 (#61) was actually #3 in reverse sequence.
And, of course, all N90 combinations were reserved for future use, even though all eight of them appear on Chart 5. Apparently, Chart 5 had been abandoned before 1995.
I assume that a major factor in the selection of new area codes after
1/1/1995 was conflict-avoidance: avoiding conflict between an area code and any central office code within the area code. This would have been a further reason for abandoning Chart 5.Even NPA 847 obeyed this constraint when it was first assigned; 847-847 appeared some time later.
-------- References -------------
[1] American Telephone and Telegraph Company, Engineering and Network Services Department, Systems Planning Section. "Notes on Distance Dialing," Section 2 ("Numbering Plan and Dialing Procedures"), 1975. [2] Ibid, Section 2, p.2. [3] Ibid, Section 2, p.17. [4] Carl Moore: history.of.area.splits. November 2, 1995.