diff --git a/.coveragerc b/.coveragerc
index e4f9d36..e265572 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -1,3 +1,18 @@
 [run]
-include = src/wormhole/*
+# only record trace data for wormhole.*
+source = wormhole
+# and don't trace the test files themselves
 omit = src/wormhole/test/*
+
+# This allows 'coverage combine' to correlate the tracing data built while
+# running tests in multiple tox virtualenvs. To take advantage of this
+# properly, use "coverage erase" before tox, "coverage run --parallel-mode"
+# inside tox to avoid overwriting the output data (by writing it into
+# .coverage-XYZ instead of just .coverage), and run "coverage combine"
+# afterwards.
+
+[paths]
+source =
+       src/
+       .tox/*/lib/python*/site-packages/
+       .tox/pypy*/site-packages/