  The Schur function indexed by a partition lambda with at most n parts is the
sum of the weight monomials for the Young tableaux of shape lambda. Let pi be
an n-permutation. We give two descriptions of the tableaux that contribute
their monomials to the key polynomial indexed by pi and lambda. (These
polynomials are the characters of the Demazure modules for GL(n).) The "atom"
indexed by pi is the sum of weight monomials of the tableaux whose right keys
are the "key" tableau for pi. Schur functions and key polynomials can be
decomposed into sums of atoms. We also describe the tableaux that contribute to
an atom, the tableaux that have a left key equal to a given key, and the
tableaux that have a left key bounded below by a given key.
