"difference between further and farther" About 14,900,000 results (0.23 seconds)
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Traditionally (and prescriptively), farther is used when referring to a distance of literal, physical movement and further is used when referring to a metaphorical distance or a degree.
I walked farther into the forest to delve further into the wonders of flora and fauna.
But Eric Clapton was not incorrect when he sang "Further on up the road, baby, just you wait and see."
The reason I bring this up is that I saw the following brochure at the bank today.
Though I grew up instilled with the traditional differences in meaning, I have recently noticed that I am becoming more accustomed to the interchangeability of farther and further when referencing distances involving actual physical movement. I barely noticed the phrasing in the brochure.
There was also a poster at the bank from the "Go further" campaign that showed a couple behind the wheel of a boat. So the brochure and the poster both illustrated physical movement while using the word further.
Look how far fur has come in its non-figurative use.
Though I will say that I have quite a hard time with the reverse interchangeability. Using farther for metaphorical distances or degrees still sounds unnatural to me.