Sternocleidomastoid (SCM) can clearly be seen when you turn your head to one side, on the opposite side of the neck.
Origin : Anterior surface of the upper sternum.
Inner part of the clavicle.
Insertion : Mastoid process (behind the ear).
Actions : Contraction on both sides: Neck flexion.
Contraction on one side only: Laterally flexes (side bends) to the same side and rotates to the other side.
Innervation : Acessory XI nerve.
Daily uses : Looking at the floor.
Looking over your shoulder.
Holding the phone between your ear and shoulder.
Example stretches : Neck rotation stretch.