The Clerk of the House of Commons is an office dating from 1363. In that year the first known Clerk, Robert de Melton, was appointed, but it is likely that Chancery Clerks would have attended meetings of the early mediaeval House of Commons long before then. They were “clerks” because they were Clerks in Holy Orders: priests, who could read and write when the generality of Members probably could not.
Until the 17th century, Clerks were closely associated with the Sovereign and Government, and were part of the core machinery for securing government business and policy. The Clerks then became impartial advisers to the House as a whole; and Paul Jodrell (Clerk from 1683 to 1727) and John Hatsell (Clerk from 1768 to 1797, and author of Precedents of Proceedings of the House of Commons) became known as eminent proceduralists. Sir Thomas Erskine May (Clerk from 1871 to 1886) is still known for the authoritative manual Parliamentary Practice which bears his name, and is now in its 24th edition.
The Clerk of the House is appointed by the Sovereign by Letters Patent. In these he is (but nowhere else) formally styled “the Under Clerk of the Parliaments, to wait upon the Commons”. Appointment by Letters Patent, in the same way as a senior Judge, is an important guarantee of the Clerk’s independence: he can be dismissed only upon an Address of both Houses to the Sovereign. This means that he is able to advise the House as he sees best, no matter how unpopular that advice might be.