In the first chapter of the fifth Perry Mason novel, The Case of the Curious Bride, Erle Stanley Gardner lays out Perry and Della's relationship thusly:
Between Della Street and Perry Mason was that peculiar bond which comes to exist between persons of the opposite sex who have spent years together in an exacting work where success can only be obtained by perfect coordination of effort. All personal relations are subordinated to the task of achievement, which brings about a more perfect companionship than where companionship is consciously sought.
(Cardinal edition, 1959, p. 5)