Sentence:  We first define a function b ( t ) as follows : given the set of <WSD>squares</WSD> such that each has three corners on C and vertex at t , b ( t ) is the corresponding set of positive parametric differences between t and the backward corner points . 
Meaning of the word marked with <WSD> tags:  (geometry) a plane rectangle with four equal sides and four right angles; a four-sided regular polygon