If there is more than one vowel and the first vowel is i, u, or ü, then the tone mark appears on the second vowel.[citation needed]
In all other cases, the tone mark appears on the first vowel.
(y and w are not considered vowels for these rules.)
The reasoning behind these rules is in the case of diphthongs and triphthongs, i, u, and ü (and their orthographic equivalents y and w when there is no initial consonant) are considered medial glides rather than part of the syllable nucleus in Chinese phonology. The rules ensure that the tone mark always appears on the nucleus of a syllable.
Another algorithm for determining the vowel on which the tone mark appears is as follows:
First, look for an "a" or an "e". If either vowel appears, it takes the tone mark. There are no possible pinyin syllables that contain both an "a" and an "e".
If there is no "a" or "e", look for an "ou". If "ou" appears, then the "o" takes the tone mark.
If none of the above cases hold, then the last vowel in the syllable takes the tone mark.
