title_body: Why is a cup of coffee not 6 oz.?
upvoted_answer: Do not blame the coffee makers for what has akways been an ambiguous meassure - or rather, not a unit for meassuring at all. Only in some areas of the world a "cup" has evolved into a fixed unit.
  
  First and foremost, a cup is a drinking vessel for usually hot beverages and usually with a handle.
  
  How big a cup is, is primarily a cultural thing. To showcase two extremes:
  
  An espresso cup 
  
  (source)
  
  vs. a jumbo coffee mug .
  
  (source) 
  
  I am from Germany. We typically use [ml] for volumetric meassurements. Whenever I come across a recipe quoting "cups" I start my research: Where did it originate? Is it from the US? I use 245 ml. From current Germany? Probably 200 ml. From my Granny's time? 150 to 180 ml probably fits the bill.
  
  Similar rues apply for coffee, from a small espresso or mokka cup in southern Europe or the Arab world to a mug of filtered coffee in the US. 
  
  There seems to be one correlation though - the "stronger" (=more concetrated) a coffe is customarily prepared, the smaller the cup. The amount of ground coffee per "serving unit" seems surprisingly constant for the various preparation styles - about a heaping teaspoon per "cup". The rest is a matter of taste.