221b Baker Street is the London address of the fictional detective Sherlock Holmes, created by Sir Arthur Conan Doyle. At the time the Holmes stories were published, the numbers in Baker Street did not go as high as 221 and so the address did not exist! However, Baker Street was later extended and, in 1932, the Abbey National Building Society moved into premises at 219–229 Baker Street. For many years, Abbey National employed a full-time secretary to answer mail addressed to Sherlock Holmes. In 1990, a blue plaque signifying 221b Baker Street was installed at the Sherlock Holmes Museum, situated elsewhere on the same block, and there followed a 15-year dispute between Abbey National and the Holmes Museum for the right to receive mail addressed to 221b Baker Street. Since the closure of Abbey House in 2005, ownership of the address by the Holmes Museum has not been challenged, despite its location between 237 and 241 Baker Street. So, if you visit Baker Street, London today, you will find, unusually for a British city, the number 221b out of sequence between 237 and 241. Why not come and see for yourself?

More Info: en.wikipedia.org