| Summary: | We determine the 1-form symmetry group for any 4d N = 2 class S theory
constructed by compactifying a 6d N=(2,0) SCFT on a Riemann surface with
arbitrary regular untwisted and twisted punctures. The 6d theory has a group of
mutually non-local dimension-2 surface operators, modulo screening.
Compactifying these surface operators leads to a group of mutually non-local
line operators in 4d, modulo screening and flavor charges. Complete
specification of a 4d theory arising from such a compactification requires a
choice of a maximal subgroup of mutually local line operators, and the 1-form
symmetry group of the chosen 4d theory is identified as the Pontryagin dual of
this maximal subgroup. We also comment on how to generalize our results to
compactifications involving irregular punctures. Finally, to complement the
analysis from 6d, we derive the 1-form symmetry from a Type IIB realization of
class S theories.
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