Its name tells you its attachments! Sterno = sternum, cleido = clavicle, mastoid = skull.

You have two! One on each side of your neck.

It can turn your head. When one SCM contracts, it helps rotate your face toward the opposite side.

Both sides can help you look up. Together, the SCMs assist with neck flexion.

It can become very noticeable. Turn your head to one side and you can often see the opposite SCM stand out along your neck.