The major radio networks were:

1. National Broadcasting Company (NBC), Red Network a development by the Radio Corporation of America (RCA), 1926
2. NBC Blue Network, launched 1927, divested under antitrust law and became the American Broadcasting Company in 1945
3. Columbia Broadcasting System (CBS), 1927
4. Mutual Broadcasting System, 1934. Mutual was run as a cooperative in which the flagship stations owned the network, not the other way around as was the case with the Big Three.

Of these four radio networks: NBC, ABC, and CBS became TV networks but Mutual ceased to exist.

Other TV networks of today (e.g. PBS) came much later and have no connection to the 1930s radio networks.