X

It is conventional among linguists to say that the letter x is best represented phonetically as the two symbols /ks/. This is true when the sound is unvoiced - especially when it follows a vowel. But that is not the whole story. When the sound is voiced, for example when the x is in initial position (at the beginning of a word) it is more of a /gz/. This is mostly found in words of Greek origin, like the writer Xenophon, the king Xerxes, or Socrates's wife Xanthippe; in English, the usual realization is the voiced sibilant -z-. (Note that in the way that the common abbreviation Xmas is pronounced, phonetically speaking it starts with a vowel and so is pronounced in the unvoiced style - IPA: /ˈɛks məs/). You may also want to see AWE's note on Xmas.