diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 571bfc33..1145ccb6 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -16,7 +16,7 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 * HDF5: a data file missing next to a VDS master now reads as the error-pixel marker instead of zero counts, so those frames are masked rather than silently integrated as blank.
 * The writer refuses a stream whose start message declares a different pixel format than its images carry, instead of writing a master that does not describe its own data.
 * A grid scan and a goniometer axis are no longer alternatives - both can be set, and the grid scan is no longer silently dropped when an axis is present.
-* HDF5: a grid-scan file now records the (stationary) spindle, so it can be opened by programs that require a rotation axis; DIALS refused the previous files outright.
+* HDF5: a grid-scan file now records the (stationary) spindle, so it can be opened by programs that require a rotation axis; DIALS refused the previous files outright. The grid stage is written as a base stage, so a grid position does not turn with the spindle.
 * The rotation axis is read back from HDF5 under whatever name it carries; only `omega` was recognised before, so a sweep recorded as e.g. `phi` re-opened as stills with nothing to say so.
 * A goniometer axis that does not turn is now kept rather than discarded, and is distinguished from a rotation sweep - it records where the head was for a still or a grid scan.
 * `images_per_file` is now chosen from the acquisition when it is not given: a rotation sweep of at most 20000 images goes into a single data file, a grid scan splits on whole fast-axis rows, and stills and serial keep 1000.
diff --git a/writer/HDF5NXmx.cpp b/writer/HDF5NXmx.cpp
index 0762b05f..525e30c8 100644
--- a/writer/HDF5NXmx.cpp
+++ b/writer/HDF5NXmx.cpp
@@ -766,9 +766,13 @@ void NXmx::Sample(const StartMessage &start, const EndMessage &end) {
     };
 
     // One chain, built from the base outwards, rather than the goniometer and the grid scan being
-    // alternatives. NXmx applies the deepest dependency first, so writing spindle -> chi -> phi ->
-    // grid translations makes the sample sit at the innermost end, which is what it physically is:
-    // the grid stage rides on the head, and the head rides on the spindle.
+    // alternatives. NXmx applies the deepest dependency first, so the order here is the mounting
+    // order: the grid stage is a BASE stage (an Aerotech xyz at SLS) that the spindle is mounted on,
+    // the spindle carries the head, and the head carries the sample. So
+    //     base -> grid -> omega -> chi -> phi -> sample
+    // and a grid position therefore does NOT turn with omega. (A head-mounted grid stage exists too
+    // - the Smargon can translate - and would sit on the other side of omega; only the base stage is
+    // modelled for now, which is what is actually used.)
     const bool write_goniometer = (end.max_image_number > 0) && start.goniometer.has_value();
     const bool write_grid_scan = start.grid_scan.has_value();
 
@@ -788,6 +792,20 @@ void NXmx::Sample(const StartMessage &start, const EndMessage &end) {
         transformations.NXClass("NXtransformations");
         hdf5_file->HardLink("/entry/sample/transformations","/entry/sample/goniometer");
 
+        // Base stage first: everything else is mounted on it. The position containers hold one entry
+        // per image and are empty if the scan stopped at the first image.
+        if (write_grid_scan && (end.max_image_number > 0)) {
+            SaveVector(transformations,"grid_scan_x", start.grid_scan->GetXContainer_m(end.max_image_number))
+                    ->Transformation("m", depends_on, "", "",
+                                     "translation", {1, 0, 0}, {0,0,0}, "");
+            depends_on = "/entry/sample/transformations/grid_scan_x";
+
+            SaveVector(transformations,"grid_scan_y", start.grid_scan->GetYContainer_m(end.max_image_number))
+                    ->Transformation("m", depends_on, "", "",
+                                     "translation", {0, 1, 0}, {0,0,0}, "");
+            depends_on = "/entry/sample/transformations/grid_scan_y";
+        }
+
         if (write_goniometer) {
             // Prefer the rotation axis refined by the offline analysis (rugnux); the broker leaves it empty
             // and the user-provided goniometer axis stands.
@@ -852,19 +870,6 @@ void NXmx::Sample(const StartMessage &start, const EndMessage &end) {
             }
         }
 
-        // The grid stage is mounted on the head, so its translations are innermost. The position
-        // containers hold one entry per image and are empty if the scan stopped at the first image.
-        if (write_grid_scan && (end.max_image_number > 0)) {
-            SaveVector(transformations,"grid_scan_x", start.grid_scan->GetXContainer_m(end.max_image_number))
-                    ->Transformation("m", depends_on, "", "",
-                                     "translation", {1, 0, 0}, {0,0,0}, "");
-            depends_on = "/entry/sample/transformations/grid_scan_x";
-
-            SaveVector(transformations,"grid_scan_y", start.grid_scan->GetYContainer_m(end.max_image_number))
-                    ->Transformation("m", depends_on, "", "",
-                                     "translation", {0, 1, 0}, {0,0,0}, "");
-            depends_on = "/entry/sample/transformations/grid_scan_y";
-        }
     }
 
     group.SaveScalar("depends_on", depends_on);