Diptych? Sounds Like an Insult to Me.
From "Shakespeare's The Tempest" by Northrop Frye in Keesey's Contexts for Criticism:
"It is clear that this point is the 'peripety' or reversal of the action, and that the play falls into the form of a diptych, the first half tragic in direction and the second half comic" (300).
What is a diptych? Well according to merriam-webster.com, it is a "2-leaved hinged tablet folding together to protect writing on its waxed surfaces." So in other words, there are two tablets. Together, these tablets make one. I found a picture of one on wikipedia's website which may aid your understanding. Frye applied this term in order to show how the two elements of the play work in a tandum. Without one, the other is meaningless.
Hmm, it’s interesting that the definition found in Merriam-Webster is different than how Frye used it. I wonder if it’s a literary term at all and if Frye just used it to mean that. Erica defined diptych too, so you might be interested in reading her blog on it: http://blogs.setonhill.edu/EricaGearhart/2009/03/what_is_a_diptych.html