diff --git a/gtsam/geometry/Similarity3.cpp b/gtsam/geometry/Similarity3.cpp
index ded917933..c76b8eb32 100644
--- a/gtsam/geometry/Similarity3.cpp
+++ b/gtsam/geometry/Similarity3.cpp
@@ -180,10 +180,11 @@ Similarity3 Similarity3::Align(const vector<Pose3Pair> &abPosePairs) {
   Point3Pairs abPointPairs;
   rotations.reserve(n);
   abPointPairs.reserve(n);
+  // note that frame "i" is the i'th object/camera/etc body frame
   Pose3 aTi, bTi;
   for (const Pose3Pair &abPair : abPosePairs) {
     std::tie(aTi, bTi) = abPair;
-    Rot3 aRb = aTi.rotation().compose(bTi.rotation().inverse());
+    const Rot3 aRb = aTi.rotation().compose(bTi.rotation().inverse());
     rotations.emplace_back(aRb);
     abPointPairs.emplace_back(aTi.translation(), bTi.translation());
   }