Sentence:  If there are W_1 "<WSD>unchecked</WSD>" whites (that is, those which enter into the solution of one clue only), and W_2 "checked" whites (those which enter into the solution of two clues), then the mean word length will be (W_1 + 2W_2)/C, where C is the number of clues. ... Acceptable (if unwelcome) difficulties arise when the initial letters of words occupy <WSD>unchecked</WSD> squares, because no hint then derives from other clues as to how that word will start. In six consecutive crossword puzzles in "w 
Meaning of the word marked with <WSD> tags:  Of a square: part of only one entry (that is, across or down, but not both).