221B Baker Street is the London address of the fictional detective Sherlock Holmes, created by author Sir Arthur Conan Doyle. In the United Kingdom, postal addresses with a number followed by a letter may indicate a separate address within a larger, often residential building. Baker Street in the late 19th century was a high-class residential district, and Holmes's apartment would probably have been part of a Georgian terrace.

A Study in Scarlet is an 1887 detective novel by British writer Arthur Conan Doyle. The story marks the first appearance of Sherlock Holmes and Dr. Watson, who would become the most famous detective duo in English literature. The book's title derives from a speech given by Holmes, a consulting detective, to his friend and chronicler Watson on the nature of his work, in which he describes the story's murder investigation as his "study in scarlet": "There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it."