The Rt. Hon. Sir Thomas Erskine May, 1st Baron Farnborough (8 February 1815–17 May 1886) was a British constitutional theorist. This derived from his career at the House of Commons, where he was appointed assistant to Thomas Vardon, chief librarian of the House of Commons Library in 1831. He became examiner of petitions for private bills in 1847, Clerk Assistant in 1856, and Clerk of the House of Commons by letters patent on 16 February 1871.

His most famous work, A Practical Treatise on the Law, Privileges, Proceedings and Usage of Parliament (known as Parliamentary Practice or simply Erskine May), is considered part of the Constitution of the United Kingdom. Similarly, it is a highly influential document in many Commonwealth nations, often with strong influence on constitutional convention.

Another notable work is The Constitutional History of England since the Accession of George III, 1760–1860 (ISBN 0-8377-2429-5).

Sir William McKay, who edited Erskine May's private journal considered May was possibly an unacknowledged son or grandson of Thomas Erskine, 1st Baron Erskine.

May was created Baron Farnborough on 11 May 1886, and died on 17 May 1886, and as he left no heirs, the title became extinct, making it the second-shortest-lived peerage in British history after the Barony of Leighton.

A bust of May is displayed in D Room of the House of Commons Library.

Parliamentary Practice can be found in various editions, including: * ISBN 0-406-29101-2 (1971 edition) * ISBN 0-406-29103-9 (1983 edition) * ISBN 0-406-97094-7 (2004 edition)

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