Sentence:  Long observation has taught Mason that most landscape can be reduced to three essential <WSD>planes</WSD> : a foreground in sharp focus - either a light area with dark accents or a dark one with lights ; a middle distance often containing the major motif ; and a background , usually a silhouetted form foiled against the sky . 
Meaning of the word marked with <WSD> tags:  (mathematics) an unbounded two-dimensional shape