  T-branes are exotic bound states of D-branes, characterized by mutually
non-commuting vacuum expectation values for the worldvolume scalars. The
M/F-theory geometry lifting D6/D7-brane configurations is blind to the T-brane
data. In this paper, we make this data manifest, by probing the geometry with
an M2-brane. We find that the effect of a T-brane is to deform the membrane
worldvolume superpotential with monopole operators, which partially break the
three-dimensional flavor symmetry, and reduce supersymmetry from N=4 to N=2.
Our main tool is 3d mirror symmetry. Through this language, a very concrete
framework is developed for understanding T-branes in M-theory. This leads us to
uncover a new class of N=2 quiver gauge theories, whose Higgs branches mimic
those of membranes at ADE singularities, but whose Coulomb branches differ from
their N=4 counterparts.
