diff --git a/image_analysis/scale_merge/SearchSpaceGroup.cpp b/image_analysis/scale_merge/SearchSpaceGroup.cpp
index fd328b0d..19d7989c 100644
--- a/image_analysis/scale_merge/SearchSpaceGroup.cpp
+++ b/image_analysis/scale_merge/SearchSpaceGroup.cpp
@@ -289,6 +289,11 @@ SearchSpaceGroupResult SearchSpaceGroup(
                 order.push_back(i);
         std::sort(order.begin(), order.end(),
                   [&](size_t a, size_t b) { return merged[a].d > merged[b].d; });   // low res -> high res
+        // A hundred reflections a shell, at most twenty-five shells - and integer division, so
+        // below two hundred there is a single shell and E^2 is I over one global mean. That
+        // leaves the operator correlation exactly where it was, Pearson being invariant to a
+        // common scale, but not the E^2 overlap cap below: it then cuts on I against the whole
+        // merge instead of against each reflection's own shell.
         const int bins = std::clamp(static_cast<int>(order.size() / 100), 1, 25);
         const size_t per = (order.size() + bins - 1) / std::max(1, bins);
         for (size_t b = 0; b * per < order.size(); ++b) {