A diamond on your back
You have two trapezius muscles, one on each side. Together they make a big diamond shape on your back.
The diamond reaches from the back of your skull, out to the tops of your shoulders, and down to the middle of your back.
It's flat and wide, like a superhero's cape lying across your upper back.
Named after a shape
Each side of the muscle has four sides, like a shape called a trapezoid. That's where the name trapezius comes from.
A trapezoid is a four-sided shape with two sides that run side by side, like train tracks. Picture the side of a bucket or a lampshade.
Lots of people call these muscles the 'traps' for short.
Shrug!
Try it: shrug your shoulders up toward your ears, like you're saying 'I don't know.' The top part of your trapezius did that.
Now squeeze your shoulder blades together, like you're trying to hold a pencil between them. That's the middle part.
The bottom part pulls your shoulder blades down. Three parts, three jobs, one muscle.
Carrying a backpack
Your trapezius helps hold your shoulders up when you carry things.
A heavy backpack hanging off one shoulder makes one side work much harder than the other. Wearing both straps shares the work.
Pack only what you need, and put the heaviest books closest to your back.