  In this draft we prove an interesting structural property related to the
problem of computing {\em all the best swap edges} of a {\em tree spanner} in
unweighted graphs. Previous papers show that the maximum stretch factor of the
tree where a failing edge is temporarily swapped with any other available edge
that reconnects the tree depends only on the {\em critical edge}. However, in
principle, each of the $O(n^2)$ swap edges, where $n$ is the number of vertices
of the tree, may have its own critical edge. In this draft we show that there
are at most 6 critical edges, i.e., each tree edge $e$ has a {\em critical set}
of size at most 6 such that, a critical edge of each swap edge of $e$ is
contained in the critical set.
