First of all, let us say that Sherlock Holmes fanon is SeriousBusiness, with the earliest examples dating to the early 1900s. The tradition of examining the stories as if they were ''really'' Dr Watson's accounts of true events, and trying to patch up his inconsistencies, anachronisms and evasions, is often referred to as "The Game". Aspects of Holmes' life that have been "deduced" by influential figures like Creator/DorothyLSayers, [[Main/FairPlayWhodunnit Ronald Knox]], [[Literature/SherlockHolmesOfBakerStreet William S Baring-Gould]] and Creator/NicholasMeyer have a tendency to show up in other people's Holmes pastiches. Some aspects have [[AscendedFanon ascended]] all the way to WordOfDante levels, being repeated without writers realising they're not part of the original Canon at all. Interestingly, it's widely thought Knox and Sayers started this kind of fan speculation initially as a parody of rationalist biblical criticism. One can only assume it took off as a hobby [[RuleOfFun because it's dreadfully fun in its own right.]]

Related to [[YMMV/SherlockHolmes YMMV]], except these particular fan-reactions and interpretations are quite widely entrenched and many have been around for upwards of fifty years and some more than a century.
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!!General Fanon
* A very popular bit of fanon in the ''Literature/SherlockHolmes'' fandom is that Dr. John H. Watson's middle name is Hamish. This theory was first devised by Creator/DorothyLSayers in order to explain why Watson's wife calls him James in one story although his first name was previously stated to be John (Hamish is the Scottish form of James).
* It's fairly established fanon that Holmes' parents were called Violet and Siger and that he at some point was part of a Shakespearean acting troupe that toured America. "Siger" is a bit of wordplay Baring-Gould invented. Holmes spent part of his "dead" years as an explorer named Sigerson - Baring-Gould retroactively made it an IncrediblyLamePun that verged on HidingInPlainSight.
* His older brother Mycroft is head of the British secret service. This sometimes assumes that the Diogenes Club is actually a front for some kind of spy organisation, an idea explored most deeply in a series of stories by Creator/KimNewman.
* The eldest of the Holmes brothers is called Sherringford or Sherrinford Holmes (the name Arthur Conan Doyle gave to Sherlock in early drafts), a country squire and RecurringFanonCharacter. Many fans assume, based on minor hints in the stories, that Sherlock comes from the landed gentry. This would imply that as well as Mycroft the civil servant (a common job for second sons), there must be a third eldest brother looking after the ancestral land.
* Irene "the Woman" Adler has a whole life outside her one appearance. Just about every non-Doyle author to write multiple Holmes stories will eventually buy into the idea that his relationship with Irene Adler was [[DatingCatwoman more than intellectual]], and that rather than the actress and (possibly) courtesan she is described as in "A Scandal In Bohemia", she was a much more [[ClassyCatBurglar modern]] and [[DarkActionGirl active]] kind of "adventuress". Common examples for this character alone include:
** That the "King of Bohemia" was actually the future Edward VII, and Watson is protecting his name. The real title of "King of Bohemia" was held by the [[UsefulNotes/TheSoundOfMartialMusic Habsburg]] Emperor of Austria, [[https://en.wikipedia.org/wiki/Franz_Joseph_I_of_Austria Franz Joseph I]], who was nearly sixty at the time of the story's publication. This one barely counts as "fanon", really, since the "King of Bohemia" would have immediately reminded readers of Prince Bertie and his many scandalous relationships with actresses and members of the ''demi-monde''.
** Almost everybody assumes that Watson was also protecting her when he refers to her as "the late Irene Adler" in the story, which he is recounting only three years after the events in 1888. Particularly since this would rule out her and Holmes meeting during his "Great Hiatus" from 1891 to '94.
** Baring-Gould had Holmes and [[PromotedToLoveInterest Irene Adler]] meet in Montenegro while he was [[FakingTheDead faking his death]] between ''The Final Problem'' and ''The Empty House'' and father a child who would grow up to be Literature/NeroWolfe (an idea first put forward by John D Clark, and expanded by John Lescroart who wrote two pastiches featuring a young Nero Wolfe under an assumed name that also [[LawyerFriendlyCameo connects more closely to Holmes]].)
* Holmes working on the UsefulNotes/JackTheRipper case is a perennial favourite. Common plots include [[TreacheryCoverup Holmes discovering the truth and being unable to reveal it]], Holmes [[MyGreatestFailure unexpectedly failing to solve it]], [[ADayInTheLimelight Watson solving it himself]], and at least one case of Holmes being the Ripper himself.
* Holmes's retirement to bee-keeping was in the hope of creating "royal jelly" (believed then to be a sort of FountainOfYouth) and that Holmes spent the last decade of his life fighting Nazis before dying at the ripe old age of ''90''.
* The idea that Watson had more than one wife comes from several stories. In "The Adventure of the Blanched Soldier", Holmes, for once narrating a story, talks about Watson having "deserted me for a wife". The story is explicitly dated to 1903, well after the death of Mary Watson née Morstan. And "A Scandal in Bohemia" and "The Five Orange Pips" both briefly mention Watson being married ''before'' his canonical meeting with Mary. There is much fannish argument about who these other two (at least) might be.
* ''The Exploits of Sherlock Holmes'', written by Arthur Conan Doyle's son, Adrian, and his editor, Creator/JohnDicksonCarr, is in [[UsefulNotes/SchrodingersCat an awkward place]] where [[BrokenBase half the fandom]] considers it fanon, and the other half considers it FanonDiscontinuity.
* There's a long-running train of thought in parts of the fandom that either Sherlock or Watson (most commonly the former) is a UsefulNotes/{{trans|gender}} man. The earliest example seems to be Creator/RexStout's essay for membership of the Baker Street Irregulars, titled "Watson Is a Woman".
* Many adaptations talking about Sherlock's family-life (''Film/YoungSherlockHolmes'', ''Literature/TheSevenPercentSolution'', ''Series/{{Sherlock}}'') seem to be agreed on the fact that a young Sherlock Holmes deduced that his father was having an affair, told his mother and ended up ruining the family. The earliest (and grimmest, given that it results in a murder-suicide) example seems to be Nicholas Meyer's ''The Seven-Per-Cent Solution''.
* For some reason, the Holmes/Watson ship is so widespread, it's often mistaken for actual {{Canon}} by fandom newbies. Adaptations don't help,as they love to tease it. Is considered SeriousBusiness by some fans to the point [[http://www.nekosmuse.com/sherlockholmes/subtext.htm essays are written about the "subtext"]]. (In a series where [[HaveAGayOldTime "ejaculation" means "sudden exclamation" and nothing else]], at that.)
* Watson's appearance is never described beyond being "brown as a nut and thin as a lath" in ''A Study in Scarlet'', and ''that'' after having just returned from severe illness abroad. Later, Watson's description was given as "middle-sized, strongly-built, square jaw, thick neck, and mustached." Nowadays, though, it's generally accepted that he was blond - helped along, perhaps, by David Burke of [[Series/SherlockHolmes the Granada series]] and Vitaly Solomin of [[Series/TheAdventuresOfSherlockHolmesAndDoctorWatson the Russian series]], not to mention Creator/JudeLaw and Creator/MartinFreeman.
* Watson's war wound is inconsistent, with some stories (including the first) claiming it was in the shoulder and others (the majority) in the leg. The obvious conclusion was that he was shot twice, in two separate places, but one piece of fanon that sprung up was that Watson was being very Victorian and delicate about where he was ''actually'' shot - in the buttocks (the "shoulder" of the leg).
* Another piece of {{Fanon}} that's spreading is the given name "Geoffrey" for Inspector ''G.'' Lestrade. This originated with Creator/MarciaWilson and has been picked up by an unknown number of fans, including [[Fanfic/DeliverUsFromEvilSeries Aleine Skyfire]] and [[Fanfic/AStudyInRegret Riandra]].
* Another perennial favourite is retroactively connecting standalone stories to Holmes' nemesis Professor Moriarty. Almost any story involving a criminal conspiracy has been "revealed" as Moriarty's scheming (or acting as a "consulting criminal") by some writer.
* Doyle's description of Moriarty's personal life was... [[ContinuitySnarl confused]], to say the least. In his debut story his brother is an army colonel named James, in "The Empty House" James has become the Professor's own name, and in a third story an unnamed brother is mentioned to be a humble country station-master. Some works [[CompositeCharacter declare both brothers to be the same person]], but others identify him as a third brother. Those stories almost always claim he is ''also'' named James Moriarty, making it a character point that all three brothers shared a first name.

!!FanNickname
* The Agent - Sir Arthur Conan Doyle; a reference to the LiteraryAgentHypothesis.
* The Master - Literature/SherlockHolmes.
** Wait, shouldn't Moriarty be [[Series/DoctorWho The Master]]?
* The Good Doctor - Watson. [[Series/DoctorWho No relation]].
** [[EpilepticTrees As far as you know.]]
** The ''Series/DoctorWho'' screenwriters [[Series/{{Sherlock}} are not helping the confusion.]]

!!Memes
* Calling someone (usually sarcastically) solving some mundane problem "Sherlock Holmes" is an OlderThanTelevision meme.
* Exclaiming "No shit, Sherlock" when someone [[CaptainObvious states the obvious.]]

!!Common FanficTropes
''Examples should be tropes that show up a lot in fanfic which are not just taken purely from canon:''

!!Adaptation-Specific
* ''{{Fanon/Sherlock}}'' (BBC series)
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[[folder: Granada TV Series]]
[[/folder]]

[[folder: Guy Ritchie film series]]
[[/folder]]

[[folder: Elementary TV Series]]
* Watson is bisexual.
* Moriarty is bisexual. Even if she's not, she's [[SingleTargetSexuality specifically into Joan]].
* Sherlock is on the autism spectrum. The show itself has even pointed out his HollywoodPsych (by a canonically autistic person at that!), but he's never been outright noted as autistic.
[[/folder]]