Just to tack on my simple opinion: 'Up' is in context to the current application. 'Back' is in context the users perspective. If I'm in the browser and click a

Just to tack on my simple opinion: 'Up' is in context to the current application. 'Back' is in context the users perspective. If I'm in the browser and click a mailto link, it opens the email client. The 'up' metaphor should return me to the inbox. The 'back' metaphor would return me to the web browser/webpage. (Did not realize I was using the exact same metaphor as my sibling, sorry)