  The lower spectral radius, or joint spectral subradius, of a set of real $d
\times d$ matrices is defined to be the smallest possible exponential growth
rate of long products of matrices drawn from that set. The lower spectral
radius arises naturally in connection with a number of topics including
combinatorics on words, the stability of linear inclusions in control theory,
and the study of random Cantor sets. In this article we apply some ideas
originating in the study of dominated splittings of linear cocycles over a
dynamical system to characterise the points of continuity of the lower spectral
radius on the set of all compact sets of invertible $d \times d$ matrices. As
an application we exhibit open sets of pairs of $2 \times 2$ matrices within
which the analogue of the Lagarias-Wang finiteness property for the lower
spectral radius fails on a residual set, and discuss some implications of this
result for the computation of the lower spectral radius.
