The Roud Folk Song Index is a database of around 250,000 references to nearly 25,000 songs collected from oral tradition in the English language from all over the world. It is compiled by Steve Roud, a former librarian in the London Borough of Croydon. Roud’s Index is a combination of the Broadside Index (printed sources before 1900) and a “field-recording index” compiled by Roud. It subsumes all the previous printed sources known to Francis James Child (the Child Ballads) and includes recordings from 1900 to 1975. […]
The primary function of the Roud Folk Song Index is as a research aid correlating versions of traditional English-language folk song lyrics independently documented over past centuries by many different collectors across (especially) the UK and North America. It is possible by searching the database, for example by title, by first line(s), or subject matter (or a combination of any of a dozen fields) to locate each of the often numerous variants of a particular song. […]
He began it in around 1970 as a personal project, listing the source singer (if known), their locality, the date of noting the song, the publisher (book or recorded source), plus other fields, and crucially assigning a number to each song, including all variants (now known as the “Roud number”) to overcome the problem of songs in which even the titles were not consistent across versions. The system initially used 3×5-inch filing cards in shoeboxes.
Source: Roud Folk Song Index – Wikipedia