When I was budding young grammar Nazi, the distinction between the modifiers further and farther was a particular peeve of mine. I cannot tell you where I first learned that farther was to be reserved for literal distances (e.g., I can’t walk any farther) and further for figurative distance or depth (e.g., Let’s discuss this further over a couple cappuccinos), but I considered it a hard and fast rule. Confusing the two was proof positive that the speaker was a nincompoop worthy of scorn.

Then I noticed that my favorite author, C.S. Lewis, habitually used further when referring to literal distance. He even did it in one of my favorite scenes — the climax of The Last Battle, when characters who have entered the heavenly new Narnia exhort the others:

I have come home at last! This is my real country! I belong here. This is the land I have been looking for all my life, though I never knew it till now…Come further up, come further in!

This left me with only two reasonable options: Either Lewis wasn’t smart enough to know the difference between further and farther, or my understanding of the distinction was in need of revision. I wisely went with the latter.

Simply, Americans make more of the distinction than the British. A number of U.S. style guides and grammar experts insist upon, or at least recommend, using farther only when talking about actual, physical distance. Further works in all other contexts. In the U.K., I understand, this usage is most common, but less strictly enforced. The words are considered synonymous and interchangeable.

These words can be used as adverbs or adjectives, but the distinction between them, if you insist upon it, applies in either case.

• Adverb: I cannot endure this Justin Bieber song any further.
• Adverb: I could toss Justin Bieber farther than you.
• Adjective: For your further enjoyment, here’s a song that’s not by Justin Bieber.
• Adjective: Justin Bieber is performing in the arena on the farther end of town.

Both words derive from the same root and both have a long history with virtually identical usage. Further is far more common on both sides of the Atlantic, most likely because it can be used as a verb as well (e.g., Let’s not further this disagreement), and because the figurative meaning is more commonly used than the literal in everyday communication.

Summary

To get to the point, I suggest that Americans stick to the distinction and reserve farther for referring to literal distance only. This will help you avoid long, boring arguments with grammar Nazis. Just don’t make a fuss when you see the words used interchangeably — especially in works by British writers.

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