I see FURTHER and FARTHER mixed up all the time out in the wild and in my clients’ manuscripts.
This is no surprise because their spelling is so similar. And honestly, I’m sure some people don’t even realize there are two different words because lots of folks use just one when speaking.
So when should you use FURTHER and when should you use FARTHER?
Use FURTHER when you’re describing a metaphorical distance.
Example: My mind wandered further down the rabbit hole.
Use FARTHER when you’re describing a measurable distance.
Example: How much farther do we have to drive?