X n.
(that of ks), as in wax; a compound vocal sound (that of gz), as in example; and, at the beginning of a word, a simple vocal sound (that of z), as in xanthic. See Guide to Pronunciation, §§ 217, 270, 271.
Y n.
Y, the twenty-fifth letter of the English alphabet, at the beginning of a word or syllable, except when a prefix (see Y-), is usually a fricative vocal consonant; as a prefix, and usually in the middle or at the end of a syllable, it is a vowel. See Guide to Pronunciation, §§ 145, 178-9, 272.