2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/54579We discuss various dual {\it pairs} of $M$-theory compactifications. Each pair consists of a compactification in which the two-brane plays the crucial role in relation to a string theory and a compactification in which the five-brane takes center stage. We show that in many examples such dual pairs are interchanged by the {\it same} duality transformation in each case.16 pages, Latex fileHigh Energy Physics - Theorytext