What is the difference between xDNA and yDNA?
yDNA came first:


  Lee &amp; Kool (2005) A New Four-Base Genetic Helix, yDNA, Composed of Widened Benzopyrimidine−Purine Pairs. J Am Chem. Soc. 127: 3332-3338


As seen in this Figure from the paper it is a DNA consisting of T and C substituted with yT and yC, widened analogues. 



xDNA came next (also from the Kool lab.):


  Lynch et al. (2006) Toward a Designed, Functioning Genetic System with Expanded-Size Base Pairs: Solution Structure of the Eight-Base xDNA Double Helix. J Am Chem. Soc. 128: 14704-14711


As seen in this Figure from the paper, xDNA consists of an 8 base system A,C,G,T,xA,xC,xG,xT.



This field is reviewed in:


  The XNA world: progress towards replication and evolution of synthetic genetic polymers
  Pinheiro &amp; Holliger (2012) The XNA world: progress towards replication and evolution of synthetic genetic polymers. Current Opinion in Chemical Biology 16:245-252