肩 is often used in compounds: 肩膀 (jiānbǎng, shoulder), 并肩 (bìngjiān, side by side), 肩负 (jiānfù, to shoulder/bear a responsibility). The idiom 并肩作战 means 'to fight shoulder to shoulder.'
Bottom flesh radical, the body-part form of 肉 graphically identical to 月. It marks 肩 as a body part — the fleshy area connecting arm to torso. Combined with the door-shape above, the whole evokes the hinge of the body. The radical anchors a huge body-part family: 腿 (leg), 肝 (liver), 脸 (face), 肚 (belly), 胖 (plump).
Top 户 is the door radical — a pictograph of a single-leaf door, half of the double-leaf 门. Here it functions ideographically: the angled, hinged shape mirrors the form of a shoulder joint, the body's own hinge between arm and torso. The whole character reads as a compound ideograph: a door-like joint covered with flesh.
