Sentence:  Some time in 1912 , Picasso cut out and folded a piece of paper in the shape of a guitar ; to this he glued and fitted other pieces of paper and four taut strings , thus creating a sequence of flat <WSD>surfaces</WSD> in real and sculptural space to which there clung only the vestige of a picture plane . 
Meaning of the word marked with <WSD> tags:  the extended two-dimensional outer boundary of a three-dimensional object