There are two English sounds which each have more than one I.T.A. letter whose main function is to write them. So whether the sound is written with the letters "c" or "k" in I.T.A. depends on the way the sound is written in standard English spelling, as also whether the sound is written with the ordinary "z" letter or with a special backwards "z" letter (which replaces the "s" of standard spelling where it represents a voiced sound, and which visually resembles an angular form of the letter "s"). The backwards "z" occurs prominently in many plural forms of nouns and third-person singular present forms of verbs (including "is").