Further vs. farther: is there a difference?

The traditional line: farther covers physical distance, how much farther to Cleveland, and further covers figurative distance and everything else, further discussion, further from the truth, and the verb sense, to further a cause. The far inside farther is the mnemonic: if actual far-ness is involved, farther.

The realistic footnote is that this distinction is a relatively recent tidiness imposed on words English used interchangeably for centuries, and it is dissolving in real time. Usage authorities note further steadily absorbing farther's territory, British English barely bothers with the split, and no reader has ever been confused by the wrong choice.

So the practical protocol: honor the distinction in polished writing because some readers still check, and when a case is ambiguous, how much further can this project slip, reach for further, which is standard everywhere. Farther is the specialist; further is the generalist, and generalists are safer hires.