We show that the definition of unipotent (resp. nilpotent) pieces for
classical groups given by Lusztig coincides with the combinatorial definition
using order relations on unipotent (resp. nilpotent) classes. In particular, we
give a map from the set of unipotent (resp. nilpotent) classes in
characteristic 2 to the set of unipotent (resp. nilpotent) classes in
characteristic 0 such that the fibers are the pieces.