diff --git a/python/gtsam/preamble.h b/python/gtsam/preamble.h
index f8e5804d0..6542343cb 100644
--- a/python/gtsam/preamble.h
+++ b/python/gtsam/preamble.h
@@ -1,3 +1,5 @@
+// Please refer to: https://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html
+// These are required to save one copy operation on Python calls
 PYBIND11_MAKE_OPAQUE(std::vector<gtsam::Key>);
 PYBIND11_MAKE_OPAQUE(std::vector<gtsam::Point2, Eigen::aligned_allocator<gtsam::Point2> >);
 PYBIND11_MAKE_OPAQUE(std::vector<gtsam::Pose3>);
diff --git a/python/gtsam/specializations.h b/python/gtsam/specializations.h
index c1114059b..e23534c85 100644
--- a/python/gtsam/specializations.h
+++ b/python/gtsam/specializations.h
@@ -1,3 +1,5 @@
+// Please refer to: https://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html
+// These are required to save one copy operation on Python calls
 py::bind_vector<std::vector<gtsam::Key> >(m_, "KeyVector");
 py::bind_vector<std::vector<gtsam::Point2, Eigen::aligned_allocator<gtsam::Point2> > >(m_, "Point2Vector");
 py::bind_vector<std::vector<gtsam::Pose3> >(m_, "Pose3Vector");