diff --git a/image_analysis/WriteReflections.cpp b/image_analysis/WriteReflections.cpp
index 15348a57..c2e10d7d 100644
--- a/image_analysis/WriteReflections.cpp
+++ b/image_analysis/WriteReflections.cpp
@@ -119,8 +119,11 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
 
     // ---------- merging statistics (_reflns overall + _reflns_shell loop) ----------
     // cc_half and r_meas are stored as fractions (0-1), which is the mmCIF convention. ISa (the
-    // Diederichs asymptotic I/sigma, 1/b of the a*sigma^2 + (b*I)^2 error model) has no standard CIF
-    // item, so it is written as a free-text pdbx value.
+    // Diederichs asymptotic I/sigma, 1/b of the a*sigma^2 + (b*I)^2 error model) and the twinning
+    // indicators below have no standard mmCIF item. They are written under the "jfjoch" reserved
+    // prefix (_reflns.jfjoch_*), the IUCr-sanctioned local-data-name extension for private items -
+    // NOT the "pdbx_" prefix, which is owned by the wwPDB PDBx/mmCIF dictionary and must not label
+    // items that dictionary does not define. (The other pdbx_ items here are genuine PDBx items.)
     const auto mult = [](const MergeStatisticsShell &s) {
         return s.unique_reflections > 0 ? static_cast<double>(s.total_observations) / s.unique_reflections : 0.0; };
     const auto compl_pct = [](const MergeStatisticsShell &s) {
@@ -137,16 +140,16 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
         out << "_reflns.pdbx_netI_over_sigmaI     " << Fmt(ov.mean_i_over_sigma, 2) << "\n";
         out << "_reflns.pdbx_Rrim_I_all           " << Fmt(ov.r_meas, 4) << "\n";
         out << "_reflns.pdbx_CC_half              " << Fmt(ov.cc_half, 4) << "\n";
-        out << "_reflns.pdbx_diffrn_ISa           " << CifStr(isa) << "   # asymptotic I/sigma (Diederichs)\n";
-        // Twinning indicators (no standard CIF item, written as free-text pdbx values like ISa above).
+        out << "_reflns.jfjoch_diffrn_ISa             " << CifStr(isa) << "   # asymptotic I/sigma (Diederichs)\n";
+        // Twinning indicators (no standard mmCIF item; same jfjoch local prefix as ISa above).
         if (twinning.l_test_pairs > 0) {
-            out << "_reflns.pdbx_L_test_mean_abs_L     " << Fmt(twinning.mean_abs_l, 3)
+            out << "_reflns.jfjoch_L_test_mean_abs_L      " << Fmt(twinning.mean_abs_l, 3)
                 << "   # Padilla-Yeates <|L|> (untwinned 0.500, perfect twin 0.375)\n";
-            out << "_reflns.pdbx_L_test_mean_L_squared " << Fmt(twinning.mean_l_squared, 3)
+            out << "_reflns.jfjoch_L_test_mean_L_squared  " << Fmt(twinning.mean_l_squared, 3)
                 << "   # <L^2> (untwinned 0.333, perfect twin 0.200)\n";
         }
         if (twinning.moment_reflections > 0)
-            out << "_reflns.pdbx_second_moment_I       " << Fmt(twinning.second_moment, 3)
+            out << "_reflns.jfjoch_second_moment_I        " << Fmt(twinning.second_moment, 3)
                 << "   # <I^2>/<I>^2 (untwinned 2.00, perfect twin 1.50)\n";
         out << "#\n";