Sentence:  First I make preliminary watercolor sketches in quarter scale ( approximately * * f <WSD>inches</WSD> ) in which I pay particular attention to the design principles of three simple values - the lightest light , the middle tone , and the darkest dark - by reducing the forms of my subject to these large patterns . 
Meaning of the word marked with <WSD> tags:  a unit of length equal to one twelfth of a foot