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The box spring is the wooden base underneath the mattress. Lay the mattress and box spring on the floor. You'll want to rule it out as the cause of the squeaking before you start working on the bed frame. Get on the mattress and move around a little bit - if it squeaks, you know the mattress is the culprit. Apply pressure to the top of the box spring and move it around. If you hear squeaking, it's probably the box spring rather than the bed frame that's causing the problem. Squeaking can occur where the posts attach to the rest of the bed frame, so try rocking each post. Try to pinpoint the exact location that the squeaking is coming from. The slats are the metal or wooden planks that stretch from one side of the bed frame to the other. They're what hold up the mattress and box spring. Apply pressure to the slats to see if they're causing the squeaking. Wood rubbing against wood will often cause squeaking.
Take the mattress and box spring off the bed frame. Check if it's the mattress that's squeaking. Check if it's the box spring that's squeaking. Rock the posts on the bed frame and listen closely. Rock the slats on the bottom of the bed frame.