The standard format has five lines:
- The punster: Knock, knock!
- The recipient: Who's there?
- The punster: a variable response, sometimes involving a name.
- The recipient: a repetition of the response followed by who?
- The punster: the punch line, which typically involves a pun-based misusage of the word set up during the response (How did you know?)
A variation of the format in the form of a children's game was described in 1929. In the game of Buff, a child with a stick thumps it on the ground, and the dialogue ensues:
- Knock, knock!
- Who's there?
- What says Buff?
- Buff says Buff to all his men, And I say Buff to you again.
In 1934, the standard knock-knock joke format was used in a newspaper humor column. That joke was:
- Knock, knock!
- Who's there?
- Rufus who?
- Rufus the most important part of your house.
A 1936 Associated Press newspaper article said that "What's This?" has given way to "Knock Knock!" as a favorite parlor game. The article also said that Knock Knock seemed to be an outgrowth of making up sentences with difficult words, an old parlor favorite.
Fred Allen's December 30, 1936 radio broadcast included a humorous wrapup of the year's least important events, including a supposed interview with the man who "invented a negative craze" on April 1: "Ramrod Dank... the first man to coin a Knock Knock."
"Knock knock" was the catchphrase of music hall performer Wee Georgie Wood, who was recorded in 1936 saying it in a radio play, but he simply used the words as a reference to his surname and did not use it as part of the well-known joke formula. The format was well known in the UK and US in the 1950s and early 1960s before falling out of favor. It then enjoyed a renaissance after the jokes became a regular part of the badinage on Rowan & Martin's Laugh-In.
In 2010, a letter from a steward (thought to be Jim Richardson) on the Nahlin steam yacht was discovered. The 16-page letter to his mother detailed life on the yacht during a 1936 Mediterranean cruise on which King Edward VIII and Wallis Simpson were passengers. The steward repeated a popular joke of the time: "Knock knock. Who's there? Edward Rex. Edward Rex who? Edward wrecks the Coronation."
In France and Quebec, the punchline is sometimes a pun on the title of a popular song, allowing the last answer to be sung:
Toc Toc! (Knock knock!)Sheila lutte finale... . (a pun on "c'est la lutte finale" (It's the final struggle), the first line of the chorus of The Internationale)
Qui est là? (Who's there?)
Sheila qui? (Sheila who?)
- Henry Bett (1929). The games of children: their origin and history. Singing Tree Press. p. 87.
- "Hee Haw News" p. 4. Rolfe Arrow. (Rolfe, Iowa). Sep. 10, 1934.
- "'Knock Knock' Latest Nutsy Game For Parlor Amusement." P. 1. Aug. 3, 1936. Titusville Herald (Pennsylvania). Byline Aug. 2. New York.
- Allen, Fred; Hample, Stuart (2001). All the Sincerity in Hollywood--: Selections from the Writings of Radio's Legendary Comedian Fred Allen. Fulcrum Pub. p. 3. ISBN 978-1-55591-154-6.
- Rees, Nigel (2006). A Word in Your Shell-like: 6,000 Curious & Everyday Phrases Explained. Collins. p. 395. ISBN 978-0-00-722087-8.
- "Wallis Simpson 'not good looking'". The Daily Telegraph. 1 November 2010. Retrieved 7 November 2015.