What is the name of the club to which Mycroft Holmes belongs?

Mycroft Holmes belongs to the Diogenes Club, a fictional gentlemen’s club created by Arthur Conan Doyle. The club is introduced in “The Adventure of the Greek Interpreter,” first published in 1893, when Sherlock and Watson visit Mycroft there.

Sherlock describes the Diogenes Club as a refuge for London’s most private and unsociable men. Members may enjoy comfortable chairs and newspapers, but they are expected not to acknowledge one another. Conversation is forbidden except in the Stranger’s Room, and repeated violations can lead to expulsion. Mycroft is one of the club’s founders.

The name alludes to Diogenes of Sinope, the ancient Greek Cynic philosopher associated with extreme independence and rejection of social conventions. Later adaptations often turn the club into a secret-service headquarters because Mycroft is connected with the British government. That espionage role is an adaptation-layer idea, however; in Doyle’s original stories, the club is primarily a silent retreat for “unclubbable” men. The Carlton, Reform, and Athenaeum are real London clubs, not the canonical answer.