Among all the intrinsic laryngeal muscles, the posterior cricoarytenoid (PCA) holds a unique position: it is the only abductor of the vocal folds. By rotating the arytenoid cartilages laterally, it widens the rima glottidis and opens the airway during inspiration, earning it the nickname the safety muscle of the larynx.
Every other adductor-group muscle does the opposite — the lateral cricoarytenoid, the transverse and oblique interarytenoids, and the thyroarytenoid all close the glottis. Tension of the cords is governed by the cricothyroid (lengthening/tensing, via the external laryngeal nerve), while the vocalis fine-tunes and shortens them.
Clinically this matters enormously: since abduction depends on a single muscle pair, bilateral recurrent laryngeal nerve injury abolishes abduction and the cords sit paramedian, threatening the airway with stridor and possible need for tracheostomy.
\[\boxed{\text{PCA} = \text{sole abductor of the vocal cords}}\]