Sentence:  A ground-joint union is made in three separate pieces and is used for joining two pipes. It consists of two machined pieces with <WSD>female</WSD> pipe threads, which are screwed on the pipes to be united, and a threaded collar which holds the two pieces of the union together. 
Meaning of the word marked with <WSD> tags:  Having an internal socket, as in a connector or pipe fitting. (from 16th c.)