Given the eigenvalues of a graph \(G\) on \(n\) vertices, for the \(i\)th eigenvalue of (a) the complement \(\overline G\) of \(G\), (b) the Seidel matrix of \(G\), and (c) a graph switching equivalent to \(G\), an interval containing this eigenvalue is determined. In addition, it is proved that the sum of all main eigenvalues of \(G\) (\(k\) in number) and of \(\overline G\) (the same number) is equal to \(n-k\).