The vocal cords are formed by the vocal ligaments (free superior edge of the cricothyroid membrane) that are covered in mucous membrane. Although crucial for phonation, the vocal folds are constantly held open at other times by the posterior cricoarytenoids for breathing. Only when speaking do the vocal folds intermittently close, vibrating and causing sound waves as air is pushed out. When closed for phonation, the cricothyroid and thyroarytenoid muscles can change cord length and therefore pitch. Coughing causes the most strain as the vocal cords must be tightly opposed to each other.