AbstractThe properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p_FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p_FS. We derive this result from high-magnetic-field transport measurements in La_1.6−x Nd_0.4Sr _x CuO₄ under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p_FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.