Alphabetic writing is not and cannot be an exact representation of the sequence of sounds or even of the sequence of distinctive sounds in the spoken forms of words and sentences. Consonant and vowel mean different things when applied to letters and to sounds, though there is, of course, much overlap. The y at the beginning of yet stands for a consonant sound; at the end of jetty, it stands for a vowel sound. In thick and thin, the sequence th represents a single sound, not a t sound followed by an h sound. In kite, the e represents no sound directly but distinguishes the vowel between k and t from the vowel in kit. These disharmonies arise from a number of causes. Economy in the use of letters is one factor. In addition, spoken forms are always changing over the centuries, whereas writing, particularly since the invention of printing, is very conservative. At one time the e at the end of words such as kite did stand for a vowel sound. This sound was lost between the 14th and 16th centuries, a time when other changes in the pronunciation of such words also occurred. The notorious Tough spellings in English, standing for different sounds and sound sequences in rough, cough, dough, plough, ought, and other such words, have arisen from historical changes that have driven spelling and pronunciation farther apart.