Published from the late 18th century, commercial directories list those who were engaged in specific trades or occupations in that area, town or city. There may also be a section for the clergy, nobility and those of "independent means".
Indexed under a particular trade or street, the directories, published by The Post Office, Pigot's, Kelly's, White's and many others, provide a way of finding an address for a tradesman around the date of a census. This often enables the genealogist to find the man and his family to learn their ages and places of birth.
Directories can be found in county record offices and local libraries. There are also large collections at the Society of Genealogists in Clerkenwell and the Guildhall Library, London.