Multiple Flag Varieties of Finite Type
  We classify all products of flag varieties with finitely many orbits under the diagonal action of the general linear group. We also classify the orbits in each case and construct explicit representatives. This generalizes the classical Schubert decompostion, which states that the GL(n)-orbits on a product of two flag varieties correspond to permutations. Our main tool is the theory of quiver representations. 
Peter Magyar, Jerzy Weyman, Andrei Zelevinsky (Northeastern   University)