852 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



telephone number; it appears in the directory and in most cities on the 

 telephone instrument itself. It is the address of the telephone in the 

 telephone network. Just as it is essential for efficient postal and delivery 

 service to have streets and house numbers clearly marked, it is important 

 for good telephone service that the telephone numbering plan be such 

 that it will be used with convenience and accuracy by the telephone 

 customer. 



A telephone number is comprised of two elements, a designation for 

 the central office to which the telephone is connected and a number 

 within the central office which identifies one particular telephone from 

 all others served by that office. If there is only one central office in the 

 city or town, the office designation is frequently omitted. A dial office is 

 designed to serve up to 10,000 numbers with a limitation of four digits. 

 Typical numbers are therefore MAin 2-1234, ADams-2345, 5-6789 and 

 3456, the office designations being MAin 2, ADams and 5 with the last 

 four digits in all cases representing the number within the central office. 



There is a wide variation in the types of numbering arrangements 

 in use today in the Bell System. This diversity arises from the fact that 

 telephone communities vary greatly with respect to the number of 

 telephones served, ranging all the way from New York City with its 

 more than three million telephones and three hundred central offices 

 to small villages and rural communities with perhaps a few score or a 

 few hundred telephones. 



In the 1920's when the Bell System embarked upon its program of 

 converting local offices to dial operation each exchange or city was in 

 general an entity unto itself. Customers dialed local calls within their 

 own city but all calls involving a toll or multi-unit charge required 

 handling by operators for timing and ticketing. There was no advantage, 

 therefore, in making a numbering plan for a given city more compre- 

 hensive than required to serve the telephones and central offices in that 

 city with a suitable allowance for the expected growth. Thus there were 

 formed a multitude of local dial communities, large and small, within 

 which customers could dial their own calls and connections between these 

 telephone communities were established by operators. 



Over the years these basic numbering plans which were originally 

 established for local dialing have in many of the cities proved inadequate 

 to furnish as many office codes as later events have shown are required. 

 This is due to a variety of causes. The station growth in many places has 

 outstripped all expectations and the number of central offices required 

 to serve this unprecedented demand for service consume many more 

 office codes than the original plans provided for. 



