Sherlock Holmes museum
221B Baker Street is the London residence of the famous literary detective Sherlock Holmes, created by author Sir Arthur Conan Doyle. In UK postal addresses a number followed by a letter may indicate a separate address within a larger, often residential building. In the Baker Street of the Sherlock Holmes Stories this would have been part of a Georgian terrace.
The site of the house — had it ever existed (see below) — has been much disputed by scholars, although the address did not exist at the time when the stories were first published in 1887.
We met next day as he had arranged, and inspected the rooms at No. 221B, Baker Street, of which he had spoken at our meeting. They consisted of a couple of comfortable bed-rooms and a single large airy sitting-room, cheerfully furnished, and illuminated by two broad windows.
(Arthur Conan Doyle, A Study in Scarlet, 1887)