The Sherlock Holmes Museum is a privately run museum in London dedicated to Sir Arthur Conan Doyle’s fictional detective, recreating the gas‑lit Victorian world of 221B Baker Street as described in the stories. Although the building physically sits between numbers 237 and 241 on Baker Street, it was granted the iconic “221B” address by the City of Westminster in 1990 when the museum opened.
 
In Conan Doyle’s canon, Holmes and Dr Watson are said to have lived at 221B Baker Street between 1881 and 1904, and the museum’s interiors are staged to match that period and those descriptions. 
 
Interesting facts...
221B was fictional when Conan Doyle wrote it. In 1887, Baker Street’s numbering only went up to about 85, so “221B” was invented for the stories and did not correspond to any real property.