title_body: Which types of beer are commonly used for half-and-half beers?
upvoted_answer: In a Half and Half (also known as a "Black and Tan" in certain locations), the primary factor will be the density of the two beers -- one beer must be of a lesser density than the other to stay afloat.  Contrasting colors are used for visual effect; otherwise, how might you know one was floating?
  
  Common recipes involve floating the less dense Guinness over a Bass, or a Guinness over a Harp Lager.  Its Guinness' particular light density that makes it a good candidate for so many half-and-half style recipes.