  We explore N=(1,0) superconformal six-dimensional theories arising from
M5-branes probing a transverse A_k singularity. Upon circle compactification to
5 dimensions, we describe this system with a dual pq-web of five-branes and
propose the spectrum of basic five-dimensional instanton operators driving
global symmetry enhancement. For a single M5-brane, we find that the exact
partition function of the 5d quiver gauge theory matches the 6d (1,0) index,
which we compute by letter counting. We finally show that S-duality of the
pq-web implies new relations among vertex correlators of qW-algebrae.
