[en] In both the π+π- and K+π- systems attempts to carry out a phase-shift analysis in the mass region below 1GeV/c2 have consistently found two distinct solutions. This ''up-down'' ambiguity has been resolved by appealing to additional data and to arguments based on continuity in the energy variable. The letter shown that in both cases the up-down ambiguity is simply the lowest-order Gersten ambiguity, that is, the complex conjugation of the ''Barrelet zero''