Prior to the divestiture of AT&T in 1984, a Baby Bell was one of the 22 Bell Operating Companies owned by AT&T. Collectively, along with AT&T Long Lines, Western Electric and Bell Telephone Laboratories, these companies were considered the Bell System Company Masthead Logo Logo until circa 1969, also current logo on company web site Logo 1969-1983 Western Electric (sometimes abbreviated WE and WECo) was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. ... Bell Telephone Laboratories or Bell Labs was originally the research and development arm of the United States Bell System, and was the premier corporate facility of its type, developing a range of revolutionary technologies from telephone switches to specialized coverings for telephone cables, to the transistor. ... The Bell System was a trademark and service mark used by the US telecommunications company American Telephone & Telegraph Company (AT&T) and its affiliated companies to co-brand their extensive circuit-switched telephone network and their affiliations with each other. ...

There were:

New England Telephone (ME, VT, NH, MA, RI)
Southern New England Telephone (CT) (Not a majority owned company)
New York Telephone
New Jersey Bell
Bell Telephone Company of Pennsylvania
Diamond State Telephone (Delaware)(subsidiary of Bell of PA)
Chesapeake & Potomac Telephone Company (Washington, DC)
Chesapeake & Potomac Telephone Company of Maryland
Chesapeake & Potomac Telephone Company of Virginia
Chesapeake & Potomac Telephone Company of West Virginia
Southern Bell (NC, SC, GA, FL)
South Central Bell (AL, MS, KY, TN, LA)
Ohio Bell
Cincinnati Bell (not a majority owned company)
Indiana Bell
Illinois Bell
Michigan Bell
Wisconsin Telephone
Southwestern Bell (TX, OK, AR, KS, MO)
Northwestern Bell (MN, ND, SD, IA, NE)
Mountain Bell (CO, NM, AZ, UT, MT, Southern ID, WY)
Pacific Northwest Bell (WA, OR, Northern ID)
Pacific Telephone & Telegraph Company (CA)
Bell Telephone Company of Nevada (subsidiary of PacTel)

The term 'Baby Bell' was used within the telephone industry to differentiate an operating company from the parent corporation (AT&T), known as "Ma Bell". Post-divestiture, these companies were grouped regionally under a holding company that was called a Regional Bell Operating Company. The press started using the term "Baby Bell" to refer to these regionals as entities unto themselves, so the usage has became blurred over the years. Map of the original and current companies. ...