A triangle on your back
Your shoulder blades are flat, thin bones on your upper back, one on each side. Each is shaped like a triangle with the point facing down.
Try it: ask a friend to rest a hand on your shoulder blade while you move your arm up and down. Can they feel it slide?
Across the back of each shoulder blade is a ridge of bone. You can feel it at the top of your back, running out toward your shoulder.
A sliding bone
Your shoulder blade isn't locked to your ribs. It rests on them and glides over them, held in place by muscles.
When you lift your arm over your head, your shoulder blade tilts and turns to help. Without it, you could only lift your arm partway.
It joins just two bones: your collarbone, at the top of your shoulder, and your upper arm bone.
A home base for muscles
About 17 muscles hold on to each shoulder blade. That's a lot for one bone!
Some of them move the shoulder blade around. Others start on it and reach out to move your arm. A group of four, called the rotator cuff, hugs the top of your arm bone and keeps it in its socket.
When you swim, throw a ball or climb, all these muscles work together.
A shallow socket
On the outside corner of each shoulder blade is a small, shallow dish. That's the socket. The round top of your upper arm bone fits there.
The dish is shallow, like a golf ball sitting on a tee. That's why your shoulder can move in more directions than any other joint in your body.
Try it: draw big circles in the air with your arm. Your shoulder lets you swing it all the way around.