This video is one of four in a series showing the larynx of an adult female with a left vocal fold paresis (incomplete paralysis). Note that the left vocal fold does move slightly, but is very limited compared to the right. The paresis results in weak glottic closure, which prevents normal vocal fold vibration. This video shows the larynx prior to a thyroplasty, that is, surgical placement of an implant that provides improved glottic closure and therefore better vibration. This series of videos provides a good demonstration of the concept of entrainment, in which the vocal folds become "entrained" in the airstream to vibrate, as long as they are close enough together, and have similar underlying muscle tone. The stroboscopy makes it clear that entrainment is often not achieved, leaving the vocal folds to vibrate separately, often at different frequencies. When the pitch is high enough, and there is enough longitudinal tension along the left vocal fold, the right vocal fold can vibrate against it with adequate regularity to a achieve a stable frequency, although the glottic closure is insufficient for much volume.