diff --git a/image_analysis/structure_refinement/RefinementGPU.cpp b/image_analysis/structure_refinement/RefinementGPU.cpp
index 97e31b7b8..a3de9c884 100644
--- a/image_analysis/structure_refinement/RefinementGPU.cpp
+++ b/image_analysis/structure_refinement/RefinementGPU.cpp
@@ -204,7 +204,7 @@ public:
     }
 
     void SetRestraints(int which, const RestraintSet &set) override {
-        engine_->SetRestraints(which, set.records, set.plane_atoms, set.slot_start, set.slot_index, set.slots);
+        engine_->SetRestraints(which, set.records, set.plane_atoms, set.slot_start, set.slot_index, set.n_slots);
     }
 
     double EvaluateRestraints(int which, const std::vector<std::array<double, 4>> &xyzb,
diff --git a/image_analysis/structure_refinement/RefinementGPU.cu b/image_analysis/structure_refinement/RefinementGPU.cu
index 4dde8f429..78d8b1e92 100644
--- a/image_analysis/structure_refinement/RefinementGPU.cu
+++ b/image_analysis/structure_refinement/RefinementGPU.cu
@@ -625,7 +625,7 @@ double RefinementGPUEngine::Evaluate(std::vector<std::array<float, 4>> *grad) {
 
 void RefinementGPUEngine::SetRestraints(int which, const std::vector<RestraintRecord> &records,
                                         const std::vector<int> &plane_atoms, const std::vector<int> &slot_start,
-                                        const std::vector<int> &slot_index, int slots) {
+                                        const std::vector<int> &slot_index, int n_slots) {
     RefinementGPUEngineImpl &e = *impl_;
     DeviceGuard guard(e.device);
     RefinementGPUEngineImpl::Restraints &r = e.restraints[which];
@@ -639,7 +639,7 @@ void RefinementGPUEngine::SetRestraints(int which, const std::vector<RestraintRe
     Ensure(r.plane_atoms, r.cap_planes, plane_atoms.size());
     Ensure(r.slot_start, r.cap_start, slot_start.size());
     Ensure(r.slot_index, r.cap_index, slot_index.size());
-    Ensure(r.contrib, r.cap_slots, static_cast<size_t>(slots));
+    Ensure(r.contrib, r.cap_slots, static_cast<size_t>(n_slots));
     Ensure(r.xyzb, r.cap_xyzb, r.atoms);
     Ensure(r.grad, r.cap_atoms, r.atoms);
     if (!r.sum)
diff --git a/image_analysis/structure_refinement/RefinementGPUEngine.h b/image_analysis/structure_refinement/RefinementGPUEngine.h
index a8b34807a..c3281301b 100644
--- a/image_analysis/structure_refinement/RefinementGPUEngine.h
+++ b/image_analysis/structure_refinement/RefinementGPUEngine.h
@@ -65,7 +65,7 @@ public:
 
     // Restraint set `which` (0 or 1), as RestraintSet holds it.
     void SetRestraints(int which, const std::vector<RestraintRecord> &records, const std::vector<int> &plane_atoms,
-                       const std::vector<int> &slot_start, const std::vector<int> &slot_index, int slots);
+                       const std::vector<int> &slot_start, const std::vector<int> &slot_index, int n_slots);
     // The sum of E of set `which` at `xyzb` (x, y, z, B per atom) and dE/d(x, y, z, B) per atom.
     double EvaluateRestraints(int which, const std::vector<std::array<float, 4>> &xyzb,
                               std::vector<std::array<float, 4>> *grad);
diff --git a/image_analysis/structure_refinement/RefinementRestraints.cpp b/image_analysis/structure_refinement/RefinementRestraints.cpp
index 3b79485ef..d1c8e698a 100644
--- a/image_analysis/structure_refinement/RefinementRestraints.cpp
+++ b/image_analysis/structure_refinement/RefinementRestraints.cpp
@@ -96,25 +96,25 @@ V3<double> FromImage(const RestraintRecord &r, V3<double> g) {
 }  // namespace
 
 void RestraintSet::Add(RestraintRecord r, int n_atoms_of_r) {
-    r.slot = slots;
-    slots += n_atoms_of_r;
+    r.slot = n_slots;
+    n_slots += n_atoms_of_r;
     records.push_back(r);
 }
 
 void RestraintSet::Append(const RestraintSet &other) {
     const int plane_offset = static_cast<int>(plane_atoms.size());
     for (RestraintRecord r : other.records) {
-        r.slot += slots;
+        r.slot += n_slots;
         if (r.kind == static_cast<int>(RestraintKind::Plane))
             r.atom[0] += plane_offset;
         records.push_back(r);
     }
     plane_atoms.insert(plane_atoms.end(), other.plane_atoms.begin(), other.plane_atoms.end());
-    slots += other.slots;
+    n_slots += other.n_slots;
 }
 
 void RestraintSet::Finalize(size_t n_atoms) {
-    slot_atom.assign(slots, -1);
+    slot_atom.assign(n_slots, -1);
     for (const RestraintRecord &r : records) {
         if (r.kind == static_cast<int>(RestraintKind::Plane)) {
             for (int k = 0; k < r.atom[1]; k++)
@@ -136,7 +136,7 @@ void RestraintSet::Finalize(size_t n_atoms) {
         slot_start[a + 1] += slot_start[a];
     slot_index.assign(slot_start[n_atoms], 0);
     std::vector<int> fill(slot_start.begin(), slot_start.end() - 1);
-    for (int s = 0; s < slots; s++)
+    for (int s = 0; s < n_slots; s++)
         if (slot_atom[s] >= 0)
             slot_index[fill[slot_atom[s]]++] = s;
 }
diff --git a/image_analysis/structure_refinement/RefinementRestraints.h b/image_analysis/structure_refinement/RefinementRestraints.h
index 0fcb13761..64b2e2ce2 100644
--- a/image_analysis/structure_refinement/RefinementRestraints.h
+++ b/image_analysis/structure_refinement/RefinementRestraints.h
@@ -26,7 +26,7 @@
 struct RestraintSet {
     std::vector<RestraintRecord> records;
     std::vector<int> plane_atoms;
-    int slots = 0;
+    int n_slots = 0;
     std::vector<int> slot_start;   // n_atoms + 1
     std::vector<int> slot_index;