Abstract. A Hermitian matrix X is called a g-inverse of a Hermitian matrix A, denoted by A − , if it satisfies AXA = A. In this paper, a group of explicit formulas are established for calculating the global maximum and minimum ranks and inertias of the difference A − − P N − P * , where both A − and N − are Hermitian g-inverses of two Hermitian matrices A and N , respectively. As a consequence, necessary and sufficient conditions are derived for the matrix equality A − = P N − P * to hold, and the four matrix inequalities A − > (≥, <, ≤) P N − P * in the Löwner partial ordering to hold, respectively. In addition, necessary and sufficient conditions are established for the Hermitian matrix equality A † = P N † P * to hold, and the four Hermitian matrix inequalities A † > (≥, < , ≤) P N † P * to hold, respectively, where (·) † denotes the Moore-Penrose inverse of a matrix. As applications, identifying conditions are given for the additive decomposition of a Hermitian g-inverse C − = A − + B − (parallel sum of two Hermitian matrices) to hold, as well as the four matrix inequalities C − > (≥, <, ≤) A − + B − in the Löwner partial ordering to hold, respectively.