The oldest surviving complete manuscript for many of the dialogues is the Clarke Plato (Codex Oxoniensis Clarkianus 39, or Codex Boleianus MS E.D. Clarke 39), which was written in Constantinople in 895 and acquired by Oxford University in 1809. The Clarke is given the siglum "B" in modern editions. "B" contains the first six tetralogies and is described internally as being written by "John the Calligrapher" on behalf of Arethas of Caesarea. It appears to have undergone corrections by Arethas himself. For the last two tetralogies and the apocrypha, the oldest surviving complete manuscript is Codex Parisinus graecus 1807, designated "A", which was written nearly contemporaneously to "B", circa 900 AD. "A" must be a copy of the edition edited by the patriarch, Photios, teacher of Arethas."A" probably had an initial volume containing the first 7 tetralogies which is now lost, but of which a copy was made, Codex Venetus append. class. 4, 1, which has the siglum "T". The oldest manuscript for the seventh tetralogy is Codex Vindobonensis 54. suppl. phil. Gr. 7, with siglum "W", with a supposed date in the twelfth century. In total there are fifty-one such Byzantine manuscripts known, while others may yet be found.