letters of the Tibetan script, to which they correspond. Letters 35-38 represent sounds that do not occur in Tibetan, and are either derived from an existing Tibetan base consonant (e.g. Letters 2 and 35 are both derived from the simple Tibetan letter KHA, but are graphically distinct from each other) or from a combination of an existing Tibetan base consonant and the semi-vowel (subjoined) letter WA (e.g. Letter 36 is derived from the complex Tibetan letter KHWA). As is the case with Tibetan, these letters have an inherent [a] vowel sound attached to them in non-final positions when no other vowel sign is present (e.g. the letter KA with no attached vowel represents the syllable ka, but with an appended vowel i represents the syllable ki). Letters 31-34 and 39 are vowels. Letters 31-34 follow the traditional order of the corresponding Tibetan vowels. Letter 39 represents a vowel quality that does not occur in Tibetan, and may be derived from the Tibetan double-E vowel sign. Unlike Tibetan, in which vowels signs may not occur in isolation but must always be attached to a base consonant to form a valid syllable, in the Phags-pa script initial vowels other than a may occur without a base consonant when they are not the first element in a diphthong (e.g. ue) or a digraph (e.g. eeu and eeo). Thus in Chinese Phags-pa texts the syllables u 吾 wú, on 刓 wán and o 訛 é occur, and in Mongolian Phags-pa texts the words ong qo chas "boats", u su nu (gen.) "water", e du -ee "now" and i hee -een "protection" occur. These are all examples of where 'o, 'u, 'e, 'i etc. would be expected if the Tibetan model had been followed exactly. An exception to this rule is the Mongolian word 'er di nis "jewels", where a single vowel sign is attached to a null base consonant. Note that the letter EE is never found in an initial position in any language written in the Phags-pa script (for example, in Tao Zongyi's description of the Old Uighur script, he glosses all instances of Uighur e with the Phags-pa letter EE, except for when it is found in the initial position, when he glosses it with the Phags-pa letter E instead). However, initial semi-vowels, diphthongs and digraphs must be attached to the null base consonant 'A (Letter 30). So in Chinese Phags-pa texts