Sentence:  <WSD>X</WSD>", "Y" and "Z" are pronounced "big-X", "big-Y", and "big-Z", or "cap-X", "cap-Y", and "cap-Z", to distinguish them from little "x" and little "y", to be described in a moment." 
Meaning of the word marked with <WSD> tags:  One of the tristimulus values which, with Y and Z, defines coordinates in a three-dimensional color space. Pronounced "big X" or "cap X".