What is decalage and how it affects airplane performance?
This article from flying magazine has a nice definition (including some history) 


  Decalage — a French word meaning “shift” or “offset” — is, broadly
  speaking, a difference between the incidences of any two lifting
  surfaces. It was originally applied to the two wings of a biplane: In
  the usual arrangement, the upper wing was farther forward than the
  lower and had a larger (can’t get away from that term!) angle of
  incidence — called positive decalage — so that it stalled first,
  shifting the center of lift aft and providing an automatic nose-down
  moment for recovery.
  
  In a monoplane, the term refers to the angles at which the wing and
  the stabilizer (or canard) are attached to the fuselage. For an
  airplane to be longitudinally stable, it must have positive
  aerodynamic decalage; roughly speaking, the forward surface must be at
  a greater angle of attack than the aft one. This principle applies to
  conventional airplanes and to canards alike.