Finding a combinatorial rule for the multiplication of Schubert polyno
mials is a long standing problem. In this paper we give a combinatoria
l proof of the extended Pieri rule as conjectured by N. Bergeron and S
. Billey, which says how to multiply a Schubert polynomial by a comple
te or elementary symmetric polynomial, and describe some observations
in the direction of a general rule. (C) 1998 Academic Press.