While the peeling–ballooning mode is known to be stabilized by strong plasma shaping in a high-confinement mode (H-mode), a significant difference is found in the roles of elongation and triangularity. The elongation mainly stabilizes the high- n ballooning branch, while the triangularity of the low- n peeling one with some destabilization of the high- n ballooning one, where n is the toroidal mode number. This difference is conjectured mainly due to the magnetic shear effect which is substantially enhanced in a high triangularity regime, giving a destabilization on the ballooning branch, while a stabilization on the peeling one. An important consequence of these different roles of the two shape parameters is that they should work together to stabilize simultaneously both branches of the PBM, implying the necessity of synergy between them for the large pedestal height enhancement.