Page "Phonation" Paragraph 12

If the vocal cords are completely relaxed, with the arytenoid cartilages apart for maximum airflow, the cords do not vibrate.

This is voiceless phonation, and is extremely common with obstruents.

If the arytenoids are pressed together for glottal closure, the vocal cords block the airstream, producing stop sounds such as the glottal stop.

In between there is a sweet spot of maximum vibration.

This is modal voice, and is the normal state for vowels and sonorants in all the world's languages.

However, the aperture of the arytenoid cartilages, and therefore the tension in the vocal cords, is one of degree between the end points of open and closed, and there are several intermediate situations utilized by various languages to make contrasting sounds.