Sentence:  As the bolt tightens into the <WSD>nut</WSD>, it pulls the tenon on the side rail into the mortise in the bedpost and locks them together. There are also some European beds that reverse the bolt and <WSD>nut</WSD> by setting the <WSD>nut</WSD> into the bedpost with the bolt inserted into a slotted area in the side of the rail. 
Meaning of the word marked with <WSD> tags:  A piece of metal, usually square or hexagonal in shape, with a hole through it having machined internal threads, intended to be screwed onto a bolt or other threaded shaft.