Sentence:  Long observation has taught Mason that most landscape can be reduced to three essential planes : a foreground in sharp focus - either a light area with dark accents or a dark one with lights ; a <WSD>middle</WSD> 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:  the part of a scene between the foreground and the background