Open where the glottis is open wide enough to allow the air stream to pass unobstructed. In this way, voiceless sounds are produced. Examples include the 'st' sequence in the word 'stone' and the 'k' in kill.

The vocal tract consists of three joined cavities: the oral cavity, the nasal cavity, and the pharyngeal cavity. The surfaces and boundaries of these cavities are known as the organs of speech. What happens to the air within these cavities is known as the oro-nasal process (Articulatory phonetics). Through the oro-nasal process we are able to differentiate between similar nasal consonants such as 'm' and 'n' and other sounds (Trujillo).

Last, but certainly not least, comes the articulation process which takes place in the mouth. In the mouth, Trujillo states that, "we can distinguish between the oral cavity, which acts as a resonator, and the articulators, which can be active...

Along the way four phases, initiation, phonation, the oro-nasal process and articulation contribute to the uniqueness of speech sounds.
Bibliography

Articulatory phonetics. http://pandora.cii.wwu.edu/vajda/ling201/test2materials/articulatory_phonetics.htm

Giegerich, H.J. (1992). English phonology: an introduction.(Cambridge Textbooks in Lingquistics) Cambridge: Cambridge University Press

Trujillo, Fernando. The production of speech sounds. http://www.ugr.es/~ftsaez/fonetica/production_speech.pdf