A famous exception to the 'phonetic spelling system' of Welsh is the use of for both /ǝ/ and the retracted high vowel /ɨ(:)/. This double use of was almost universally adopted by c. 1330, when a grammarian labelled /ǝ/ and /ɨ/ as 'dark y' and 'clear y' and illustrated them with polysyllables such as ystyr /ˈǝstɨr/ 'meaning', in which the value of was predictable from the position of in the word. At that time the three-way system of < i> for /i(:)/, 'dark' , and 'clear' was two centuries old, being first attested in Braint Teilo ('The Privilege of St Teilo'), c. 1130. Yet the 'Teilo' system is rarely attested before c. 1300; instead all three phonemes might be represented by < i> , as commonly before 1100, or by ; or might be used for /ǝ/ and/or for /ɨ(:)/, as had sometimes occurred in Old Welsh as well. This article argues that one reason, apart from scribal conservatism, for the delay in adopting the 'Teilo' system was its failure to distinguish the value of in proclitics such y /ǝ/ 'the' and y /ɨ/ 'his/her' and 'to'. For this the ultimately abortive 'Caligula' system (c. 1250) had offered a solution.1 Latin , in words of Greek origin like syllaba, was known to early Welsh scribes, as noted by Kitson (2003: 53, n. 17), although he himself favours Old English as the source of Welsh , like Charles-Edwards & Russell (1994: 422). The latter would certainly have been known in Wales; see LL 266 where Archbishop Cynesige of York (fl. 1056) is called Cy nisi. This is by Hand B, identified as Bishop Urban (Gwrgan) of Llandaf (d. 1134) by Davies (1998: 5 n. 25).