The consonants W and Y are very similar to vowel sounds. They are always regarded as consonants on functional grounds that are they are always used, like consonants, at word and syllable boundaries. Where vocal tone is concerned, consonants are to be regarded as a necessary evil. Conclusive proof of what is required of the consonants where audibility is concerned. The degree of aspiration varies considerably from speaker to speaker. It should never be prominent to a marked degree, and to this end the articulating organs must be separated neatly and with rapidity. Glottal stop is an explosive which is often heard but which is never indicated in the spelling of a word. It forms in the larynx by a closure of the vocal cords, much as a P is formed by the closure of the lips. It is used to replace the other explosive consonants, especially P, T, and K, and marks characteristic of a number of dialects.