Sentence:  Moreover , if * * f and * * f are two planes <WSD>intersecting</WSD> in a line l , tangent to Q at a point P , the two free intersections of the image curves * * f and * * f must coincide at P ' , the image of P , and at this point * * f and * * f must have a common tangent l ' . 
Meaning of the word marked with <WSD> tags:  meet at a point