The quantised charges x of four dimensional stringy black holes may be assigned to elements of an integral Freudenthal triple system whose automorphism group is the corresponding U-duality and whose U-invariant quartic norm ∆(x) determines the lowest order entropy. Here we introduce a Freudenthal duality x →x, for whichx = −x. Although distinct from U-duality it nevertheless leaves ∆(x) invariant. However, the requirement thatx be integer restricts us to the subset of black holes for which ∆(x) is necessarily a perfect square. The issue of higher-order corrections remains open as some, but not all, of the discrete U-duality invariants are Freudenthal invariant. Similarly, the quantised charges A of five dimensional black holes and strings may be assigned to elements of an integral Jordan algebra, whose cubic norm N (A) determines the lowest order entropy. We introduce an analogous Jordan dual A ⋆ , with N (A) necessarily a perfect cube, for which A ⋆⋆ = A and which leaves N (A) invariant. The two dualities are related by a 4D/5D lift.