Area codes 610 and 484
Area codes 610 and 484 are telephone area codes which serve the eastern and southeastern regions of Pennsylvania. The area includes areas to the west of Philadelphia, along with the cities of Allentown, Bethlehem, and Reading. It includes much of the Delaware Valley, including most of the Philadelphia Main Line.
Created in anticipation of number exhaustion in the area code 215 serving the whole of southeast Pennsylvania, area code 610 was split off on the morning of January 8, 1994, with a year-long permissive dialing period ending on the morning of January 7, 1995. It was Pennsylvania's first new area code since the implementation of the area code system in 1947.
Three prefixes which would have gone into 610 were instead switched to 717, the area code for most of the eastern half of the state outside of the Delaware and Lehigh Valleys. They were 267 at Denver, 445 at Terre Hill, and 484 at Adamstown, with 267 being replaced by 717-336 because 717-267 was already in use at Chambersburg. These exchanges were all served by non-Bell telephone companies which sought to consolidate their service in one area code, and would have had to change area codes anyway.
Further growth in the region over the subsequent five years spurred the introduction of overlay plans in the Commonwealth, with area code 484 being overlaid upon 610 on June 5, 1999, along with the introduction of mandatory ten-digit dialing.
Area code 835 was to be an additional overlay onto 610 and 484, announced by the Pennsylvania Public Utility Commission in May 2000 and to be implemented in 2001, but newly developed, more efficient number pooling measures were introduced instead, eliminating the immediate need for the new area code. The Commission formally withdrew plans for the new code on June 23, 2005, although the code remains reserved for later use within the Commonwealth if necessary.
Area code 610 holds the distinction of being the last area code number to be assigned that had a "1" (one) for its middle digit. When area codes were introduced in 1947, all numbers followed the pattern N0X or N1X (where the middle digit was either a zero or one). By 1994, area code 610 was the last remaining number in this group. The next new area codes after 610 to be assigned (334 in Alabama and 360 in Washington, both in 1995) were the first to have numbers other than 0 or 1 for their middle digits.
Counties served 
- Berks County - All but extreme western portions (in 717), extreme northern portions (in 570) and Hereford area (served by the Pennsburg exchange in the 215/267 area code).
- Bucks County - Northeastern portion served by the Coopersburg, Springtown, Riegelsville and Ferndale exchanges.
- Chester County - All but the extreme western portion [which is served by the (717-442) Gap exchange].
- Delaware County - All
- Lancaster County - Far southeastern portion of the county only, centered around the town of Christiana. Rest of county uses the 717 area code.
- Lebanon County - Newmanstown is the only Lebanon County municipality to use 610 as its area code, specifically 610-589-XXXX (also shared with Womelsdorf, Berks County).
- Lehigh County - All but extreme southwestern portion, served by the Pennsburg exchange in the 215/267 overlay.
- Montgomery County - Southern and western portions; other areas are served by the 215/267 area codes.
- Northampton County - All but extreme northeastern portions, such as Portland and Upper Mount Bethel Township, which are in the 570 area code
- Carbon County - Far southern portion
- Monroe County - Far southwestern portion
- Schuylkill County - Far southern portion
Prior usage for TWX 
The 610 area code (along with area codes 510, 710, 810, and 910) was used prior to 1981 by AT&T for their TWX, or TeletypeWriter eXchange network. The area code was decommissioned for TWX usage in 1981 when Western Union, who had acquired the TWX network in 1969 from AT&T and renamed it Telex II, upgraded the network to "4-row" ASCII operation (it previously used both "3-row" Baudot and ASCII transmission).
See also 
- North American Numbering Plan Administration
- NANPA Area Code Map of Pennsylvania
- List of exchanges from AreaCodeDownload.com, 484 Area Code
- List of exchanges from AreaCodeDownload.com, 610 Area Code
|Pennsylvania area codes: 215, 267, 412, 484, 570, 610, 717, 724, 814, 878|
|West: 717||610/484||East: 215/267, 856, 609, 908|
|South: 302, 410/443, 856|
|Delaware area codes: 302|
|Maryland area codes: 240, 301, 410, 443|
|New Jersey area codes: 201, 551, 609, 732, 848, 856, 862, 908, 973|