This is an Olde English locational name which derives from the pre 7th Century 'Cu' meaning 'A Cow' and 'dun', a valley pasture. There is a possiblity that 'key or kay' may be a transposition of the personal name 'Caega', but this is unlikely as 'Caega' is almost always found with 'ham' to give 'Caega's home' or farmstead. It is possible that a place called 'Kaydon or Keyden' once existed, but if so, this is apparently no longer the case. The name development includes Janet Keyden, christened at St. Vincents Church, London in 1820 and James Keydon, the brother of Janet, also christened at St. Vincents on June 5th, 1825. The first recorded spelling of the family name is shown to be that of Bartholomew Keydon, which was dated 1638, married Lucian Beale at Kingsdon, Somerset, during the reign of King Charles I, 'The Martyr' 1625 - 1649. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.