Many of the Sherlock Holmes stories, such as "The Adventure of the Speckled Band," are set during the time that Dr. Watson and Sherlock Holmes were roommates at 221B Baker Street in London. And while Watson moves out of 221B Baker Street after he marries, Holmes continues to reside there, and several of the later stories begin at or involve this now iconic address.
Holmes and Watson first meet and take up residence at 221B Baker Street in the novel A Study in Scarlet. The novel chronicles the meeting of Holmes, who was looking for a roommate, and Watson, who was also looking for a roommate and was introduced to Holmes by a friend, and then the first case on which Watson worked with Holmes. The early Sherlock Holmes short stories, as well as the most famous of the novels, The Hound of the Baskervilles, are set during the time (1881 - 1904) when the two were roommates at 221B Baker Street.
Interestingly, at the time Sir Arthur Conan Doyle wrote the Sherlock Holmes short stories and novels, 221B Baker Street did not exist; the then existing numbering system did not go that high. Subsequent expansions of the numbering system created a block that would have included 221 Baker Street, but there was no exact matching address. In 1990, the address was assigned to the Sherlock Holmes museum, although the museum actually is located between 237 and 241 Baker Street.