Further vs. Farther

What is the difference between further and farther. Is farther even a word? Can my friend throw the ball farther than me, or further than me?

ANSWER PERSON RESPONDS: Depends on how fur you want Answer Person to takes this. Well, beyond that snide jab at southern culture, demonstrating a variance that doesn’t work, there really isn’t much difference. They are both words, coming from different Middle English roots, each with different German ancestries, long used interchangeably as adverbs and adjectives. The “American Heritage® Book of English Usage” concurs with this, though it goes on to point out “A relatively recent rule, however, states that farther should be reserved for physical distance and further for nonphysical, metaphorical advancement.” [http://www.bartleby.com/64/C003/0121.html] So, unless you and your friend are throwing a virtual ball, “farther” is the way to go. Need I take this any further?