Sentence:  On a crossword puzzle grid, you shouldn't encounter any <WSD>unchecked</WSD> squares – white squares that are used in an Across entry but not in a Down entry, or vice versa. If <WSD>unchecked</WSD> squares were allowed, that would make your life harder – you'd have only one opportunity to figure out what belongs in that square. Instead, you always get two chances to fill a square: by solving the Across clue or the Down clue. 
Meaning of the word marked with <WSD> tags:  Of a square: part of only one entry (that is, across or down, but not both).