WHAT IT MEANS: Doyle's story "The Sign of Four" involved a murder victim discovered in a room locked from the inside, a killing committed by a man less than four feet in height.
WHAT WE SEE: While Sherlock is speaking to several women online, he dismisses John's reminder that he hasn't eaten.
WHAT IT MEANS: In Doyle's original stories, Sherlock would sometimes fast when he wanted to focus entirely on solving a case, saying he couldn't waste any valuable energy on digestion.
WHAT WE SEE: Just as he's about to toast the happy couple, Sherlock realizes that John's middle name Hamish provides a vital clue.
WHAT IT MEANS: In Doyle's first Sherlock Holmes story "A Study in Scarlet," Watson's middle initial was given as H but never explained. In "The Adventure of the Man with the Crooked Lip," Mary called Watson "James." To explain this error, some fans concluded that it revealed John's middle name was "Hamish," an alternate form of "James."
WHAT WE SEE: As Sherlock recalls trying to guess John's middle name, we see a flashback of him putting cigarettes inside one of his slippers.
WHAT IT MEANS: Doyle's story "The Musgrave Ritual" said that Holmes kept his tobacco in a Persian slipper on the mantel.
WHAT WE SEE: While thinking of John's middle name, Sherlock remembers that "The Woman" knew it.
WHAT IT MEANS: "The Woman" is Irene Adler, who appeared in the season 2 premier "A Scandal in Belgravia." In Doyle's stories, she outsmarted Holmes in "A Scandal in Bohemia" and then left for America with her husband. Despite his general distrust of women, Holmes developed a great admiration for Adler, keeping her photograph in a drawer and only referring to her afterward as "The Woman," for she eclipsed all other females.
WHAT WE SEE: As he considers the case, Sherlock and a mental avatar of Mycroft agree that coincidence shouldn't be trusted, since "the universe is rarely so lazy."
WHAT IT MEANS: In Doyle's "A Case of Identity," Sherlock told Watson that he saw coincidences as part of larger, complex chains of events that proved "life is infinitely stranger than anything which the mind of man could invent."
WHAT WE SEE: When Sherlock declares, "Vatican cameos!" John immediately understands that someone is in danger of being killed.
WHAT IT MEANS: Sherlock used this phrase to signal immediate threat of death in the season 2 episode "A Scandal in Belgravia." It is a reference to one of the untold tales of Sherlock Holmes. In Doyle's "The Hound of the Baskervilles," Holmes mentioned working for the Pope in a case involving Vatican cameos. Some fan websites have wrongly stated that the phrase is military slang from World War II.
WHAT WE SEE: John asks why Sherlock doesn't remember Sholto's room number and the detective protests that he'd have to delete some information in his brain.
WHAT IT MEANS: In Doyle's original story "A Study in Scarlet," and in the season 1 episode "The Great Game," Holmes told Watson that he made sure to forget information he deemed irrelevant to his work, such as that the Earth revolved around the sun and not vice versa.
