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How to Remember When to Use 'Further' Versus 'Farther'

The English language is full of words that sound similar and mean similar things, but do retain their own special place if we want to use them precisely. Who and whom. Affect and effect. Sympathy and empathy. Utmost and upmost. And the especially vexing lay, lie, lain, and laid. Another that tends to trip people up is the oh-so-similar further versus farther.