In 1984, U.S. District Judge Harold Greene issued a decision that led to the divestiture of local telco properties from AT&T and the creation of the seven "Regional Bell Operating Companies" from 22 Bell operating companies. The seven RBOCs and the original Bell companies which made them up were:
Pacific Telesis (PacTel): Pacific Telephone & Telegraph Company, Bell Telephone Company of Nevada.
Ameritech: Illinois Bell Telephone Company, Indiana Bell Telephone Company, Michigan Bell Telephone Company, The Ohio Bell Telephone Company, Wisconsin Telephone Company.
Nynex: The New York Telephone Company, New England Telephone & Telegraph Company.
Bell Atlantic: New Jersey Bell Telephone Company, Chesapeake & Potomac Telephone Company, Chesapeake & Potomac Telephone Company of Maryland, Chesapeake & Potomac Telephone Company of Virginia, Chesapeake & Potomac Telephone Company of West Virginia, The Bell Telephone Company of Pennsylvania.
Southwestern Bell: Southwestern Bell Telephone Company.
BellSouth: South Central Bell Telephone Company, Southern Bell Telephone & Telegraph Company.
U.S. West: Mountain States Telephone & Telegraph Company, Northwestern Bell Telephone Company, Pacific Northwest Bell Telephone Company, Diamond State Telephone Company.
Nynex merged with Bell Atlantic in 1997.
Bell Atlantic merged with GTE in 2000 to become Verizon (spinning off its Internet business--the former Genuity and BBN Planet--as Genuity).
Southwestern Bell acquired PacTel in 1997 and started using the name SBC, and then acquired Ameritech in 1999.
U.S. West was acquired by Qwest in 2000.
SBC acquired AT&T in 2005, and took on its name.
Most of this history is recounted in more detail, with maps and logos, here.