The (dubiously official) portmanteau couple name for the slash couple of Sherlock Holmes and John Watson, most commonly used in the fandom surrounding the BBC miniseries modernized version of the consulting detective and his medical partner.
A fanfiction written for the fandom of Sherlock Holmes, that consists of two hundred and twenty one words, the last word beginning with a "b." The name refers to the apartment shared by Sherlock and Dr. John Watson, at the address 221B Baker Street, in London. Particularly common in the fanfiction based upon the 2010&2012 BBC series, because the modernization of the setting allows interpretation of a 221B as a text message or overly long tweet.
I just wrote a slash fic, Sherlock/Watson, in the 221B format.