Let   $A$  and
 $A$  and   $B$  be
 $B$  be   $n\,\times \,n$  complex Hermitian (or real symmetric) matrices with eigenvalues
 $n\,\times \,n$  complex Hermitian (or real symmetric) matrices with eigenvalues   ${{a}_{1}}\,\ge \,\cdots \,\ge \,{{a}_{n}}$  and
 ${{a}_{1}}\,\ge \,\cdots \,\ge \,{{a}_{n}}$  and   ${{b}_{1}}\,\ge \,\cdots \,\ge \,{{b}_{n}}$ . All possible inertia values, ranks, and multiple eigenvalues of
 ${{b}_{1}}\,\ge \,\cdots \,\ge \,{{b}_{n}}$ . All possible inertia values, ranks, and multiple eigenvalues of   $A\,+\,B$  are determined. Extension of the results to the sum of
 $A\,+\,B$  are determined. Extension of the results to the sum of   $k$  matrices with
 $k$  matrices with   $k\,>\,2$  and connections of the results to other subjects such as algebraic combinatorics are also discussed.
 $k\,>\,2$  and connections of the results to other subjects such as algebraic combinatorics are also discussed.