Sentence:  Bicycle gear-sets he had once used as the basis of the design for the Camden Cycly Company plant hung on a rope in one corner , and over his desk , next to several old and dusty hats , was a clean <WSD>pair</WSD> of roller skates which he occasionally used up and down front of his house . 
Meaning of the word marked with <WSD> tags:  a set of two similar things considered as a unit