My understanding is that "further" and "farther" can be used interchangeably when talking about distances (so "further" can be used wherever "farther" can, but

My understanding is that "further" and "farther" can be used interchangeably when talking about distances (so "further" can be used wherever "farther" can, but not the other way around). This is for example what the usage note at http://www.merriam-webster.com/dictionary/farther http://www.merriam-webster.com/dictionary/farther says, although they state that the usage is becoming more polarized.