  Switches are operations which make local changes to the edges of a graph,
usually with the aim of preserving the vertex degrees. We study a restricted
set of switches, called triangle switches. Each triangle switch creates or
deletes at least one triangle. Triangle switches can be used to define Markov
chains which generate graphs with a given degree sequence and with many more
triangles (3-cycles) than is typical in a uniformly random graph with the same
degrees. We show that the set of triangle switches connects the set of all
$d$-regular graphs on $n$ vertices, for all $d\geq 3$. Hence, any Markov chain
which assigns positive probability to all triangle switches is irreducible on
these graphs. We also investigate this question for 2-regular graphs.
