title_body: 'X' marks to indicate quality?
upvoted_answer: X's and K's actually derive from turn of the last century British brewing practices where the letters and how often they were repeated denoted the basic style and relative strength of the beer.  See more here.
  
  The more in depth answer is that back then malting and kilning practices weren't as good so you generally just ended up with "Malt", which was kindof brownish but varied wildly.  So you just sort of made "Beer" out of it.  X Ales were table beer, made to be consumed relatively fresh, and K Ales were keeping beer made to be kept.  Hopping rates likely differed as well as starting strength.
  
  I think at some point later X came to denote "mild" ale, which referred to a lower hopping rate and K/AK/etc came to refer to "bitter", but the blogger I linked to above knows more about that progression than I ever will so I may be totally wrong.