Sentence:  First I make preliminary watercolor sketches in quarter scale ( approximately * * f inches ) in which I pay particular attention to the design <WSD>principles</WSD> 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 basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct