The Tet Corporation is a business organization created for three purposes: to guard the Rose, on Second Avenue and Forty-Sixth Street, to protect Stephen King, and to thwart the growth of the Sombra Corporation and North Central Positronics. Moses Carver served as president of the company from its beginning in 1977 up to 1997 when he passed the title onto his daughter, Marian Carver. In 1999, the company was worth almost ten billion dollars.
It began when Calvin Tower sold the vacant lot to Eddie Dean and Roland Deschain, under the name of Tet. The Tet Corporation didn't really get started until John Cullum met with Moses Carver and Aaron Deepneau to make it official. Carver folded the assets of Holmes Dental into the Tet Corporation to provide the business with its starting capital. Presumably, Tet made much of its money off of the computer boom in the 80s and 90s, based off of the hint given by Eddie Dean to Cullum.
The Tet Corporation built its headquarters around the Rose. The address was changed to 2 Hammarskjöld Plaza. The building itself even earned the nickname of "The Dark Tower" as a result of its dark coloration.
Tet established a ranch that employed a dozen telepaths and precognitives in Taos, New Mexico. The idea came from Calvin Tower, who had discovered the idea in a book written under the name, Daniel Holmes. Along with these psychics, Tet also employed a group called the "Calvins", in honor of Tower. These Calvins' sole job was to read the works of Stephen King and scour it for references to the Keystone World and real people.