The cords, which are attached to the cartilages, are to the human speech organs what the two vibrating reeds are to a clarinet or the strings to a violin.  They are capable of at least three distinct types of movement, each of which is of the greatest importance for speech.  They may be drawn towards or away from each other, they may vibrate like reeds or strings, and they may become lax or tense in the direction of their length.  The last class of these movements allows the cords to vibrate at different “lengths” or degrees of tenseness and is responsible for the variations in pitch which are present not only in song but in the more elusive modulations of ordinary speech.  The two other types of glottal action determine the nature of the voice, “voice” being a convenient term for breath as utilized in speech.  If the cords are well apart, allowing the breath to escape in unmodified form, we have the condition technically known as “voicelessness.”  All sounds produced under these circumstances are “voiceless” sounds.  Such are the simple, unmodified breath as it passes into the mouth, which is, at least approximately, the same as the sound that we write h, also a large number of special articulations in the mouth chamber, like p and s.  On the other hand, the glottal cords may be brought tight together, without vibrating.  When this happens, the current of breath is checked for the time being.  The slight choke or “arrested cough” that is thus made audible is not recognized in English as a definite sound but occurs nevertheless not infrequently.[14] This momentary check, technically known as a “glottal stop,” is an integral element of speech in many languages, as Danish, Lettish, certain Chinese dialects, and nearly all American Indian languages.  Between the two extremes of voicelessness, that of completely open breath and that of checked breath, lies the position of true voice.  In this position the cords are close together, but not so tightly as to prevent the air from streaming through; the cords are set vibrating and a musical tone of varying pitch results.  A tone so produced is known as a “voiced sound.”  It may have an indefinite number of qualities according to the precise position of the upper organs of speech.  Our vowels, nasals (such as m and n), and such sounds as b, z, and l are all voiced sounds.  The most convenient test of a voiced sound is the possibility of pronouncing it on any given pitch, in other words, of singing on it.[15] The voiced sounds are the most clearly audible elements of speech.  As such they are the carriers of practically all significant differences in stress, pitch, and syllabification.  The voiceless sounds are articulated noises that break up the stream of voice with fleeting moments of silence.  Acoustically intermediate between the freely unvoiced and the voiced sounds are a number of other characteristic types of voicing, such as murmuring and whisper.[16] These and still other types of voice are relatively unimportant in English and most other European languages, but there are languages in which they rise to some prominence in the normal flow of speech.