  We ask when a finite set of t-structures in a triangulated category can be
`averaged' into one t-structure or, equivalently, when the extension closure of
a finite set of aisles is again an aisle. There is a straightforward, positive
answer for a finite set of compactly generated t-structures in a big
triangulated category. For piecewise tame hereditary categories, we give a
criterion for when averaging is possible, and an algorithm that computes
truncation triangles in this case. A finite group action on a triangulated
category gives a natural way of producing a finite set of t-structures out of a
given one. If averaging is possible, there is an induced t-structure on the
equivariant triangulated category.
