Sentence:  “Quadrats, sir. We call 'em <WSD>quads</WSD>.” . . . <WSD>Quads</WSD> are the spaces left between the paragraphs that come white on the paper. If you look here, at this page that is set-up, you will see that they are deeper than the spaces left between the words and letters—regular little trenches. 
Meaning of the word marked with <WSD> tags:  A blank metal block used to fill short lines of type.