Farther vs Further
Far: farther - farthest or further - furthest?
Both forms can be used of distance:
- the farthest / furthest town
- the farthest / furthest of all destinations
but
further / furthest - with abstract nouns also means "additional / extra":
- further supplies
- further negotiations
- further discussion
- further debate
- further enquiries
- further delays
- further demands
- further information
- further instructions
- the furthest point in the discussion
- the furthest concession, etc.