We prove thatwhere P and Q are orthogonal projections on Hilbert spaces H , resp. K and X is a bounded linear operator between K and H , generalizing the well-known Akhiezer-Glazman equality (the case K = H and X = 1 H ). Specializing, we obtain results by Z. Boulmaarouf, M. Fernandez Miranda and J.-Ph. Labrouse, T. Kato or S. Maeda. We extend a theorem of Y. Kato and give some sufficient conditions for the equality P X(1 − Q) = (1 − P )XQ . We generalize results by D. Buckholtz, J.J. Koliha and V. Rakočević or S. Maeda and provide several necessary and/or sufficient conditions for the (left, right) invertibility of P X − XQ. Certain counter-examples show the consistency of the theory. Mathematics Subject Classification (2000). Primary 47A05; Secondary 47A30, 46C05, 46C07.