diff --git a/src/mol-geo/representation/structure/visual/polymer-trace-mesh.ts b/src/mol-geo/representation/structure/visual/polymer-trace-mesh.ts
index f7a75b4eb3db6037af5dca91910f0e1bccf679e0..bae42eab982fc11273ff71b67ace2cd0dfb7c0a1 100644
--- a/src/mol-geo/representation/structure/visual/polymer-trace-mesh.ts
+++ b/src/mol-geo/representation/structure/visual/polymer-trace-mesh.ts
@@ -134,7 +134,7 @@ export function PolymerTraceVisual(): UnitsVisual<PolymerTraceProps> {
             const { objectId, instanceId, elementId } = pickingId
             if (renderObject.id === objectId) {
                 const unit = currentGroup.units[instanceId]
-                const indices = OrderedSet.ofSingleton(elementId as StructureElement.Index);
+                const indices = OrderedSet.ofSingleton(elementId as StructureElement.UnitIndex);
                 return StructureElement.Loci([{ unit, indices }])
             }
             return EmptyLoci
diff --git a/src/mol-geo/representation/structure/visual/util/element.ts b/src/mol-geo/representation/structure/visual/util/element.ts
index 95b0ab5a8b38ff0eb8a8fad53070821f2f65144f..50a44321e32420b54cf80f80a4ab28f2cbc140e7 100644
--- a/src/mol-geo/representation/structure/visual/util/element.ts
+++ b/src/mol-geo/representation/structure/visual/util/element.ts
@@ -96,7 +96,7 @@ export function getElementLoci(id: number, group: Unit.SymmetryGroup, pickingId:
     const { objectId, instanceId, elementId } = pickingId
     if (id === objectId) {
         const unit = group.units[instanceId]
-        const indices = OrderedSet.ofSingleton(elementId as StructureElement.Index);
+        const indices = OrderedSet.ofSingleton(elementId as StructureElement.UnitIndex);
         return StructureElement.Loci([{ unit, indices }])
     }
     return EmptyLoci
diff --git a/src/mol-model/structure/query/selection.ts b/src/mol-model/structure/query/selection.ts
index fe4ed816818eac459d2469f75a5f79a5002c763d..72ab29106a7b81a30cef2ebcac96c8a83ebb17ac 100644
--- a/src/mol-model/structure/query/selection.ts
+++ b/src/mol-model/structure/query/selection.ts
@@ -36,12 +36,12 @@ namespace Selection {
     }
 
     export function toLoci(sel: Selection): StructureElement.Loci {
-        const loci: { unit: Unit, indices: OrderedSet<StructureElement.Index> }[] = [];
+        const loci: { unit: Unit, indices: OrderedSet<StructureElement.UnitIndex> }[] = [];
         const { unitMap } = sel.source;
 
         for (const unit of unionStructure(sel).units) {
             if (unit === unitMap.get(unit.id)) {
-                loci[loci.length] = { unit, indices: OrderedSet.ofBounds(0 as StructureElement.Index, unit.elements.length as StructureElement.Index) };
+                loci[loci.length] = { unit, indices: OrderedSet.ofBounds(0 as StructureElement.UnitIndex, unit.elements.length as StructureElement.UnitIndex) };
             } else {
                 loci[loci.length] = {
                     unit,
diff --git a/src/mol-model/structure/structure/element.ts b/src/mol-model/structure/structure/element.ts
index 1bbaf84b62041f450ce646d0f531f941c1bf39b1..f8e35caff577b023e4d64ef5c19abf1822ac3bc3 100644
--- a/src/mol-model/structure/structure/element.ts
+++ b/src/mol-model/structure/structure/element.ts
@@ -17,10 +17,11 @@ interface StructureElement<U = Unit> {
 namespace StructureElement {
     export function create(unit?: Unit, element?: ElementIndex): StructureElement { return { unit: unit as any, element: element || (0 as ElementIndex) }; }
 
+    // TODO: when nominal types are available, make this indexed by UnitIndex
     export type Set = SortedArray<ElementIndex>
 
     /** Index into Unit.elements */
-    export type Index = { readonly '@type': 'element-index' } & number
+    export type UnitIndex = { readonly '@type': 'structure-element-index' } & number
 
     export interface Property<T> { (location: StructureElement): T }
     export interface Predicate extends Property<boolean> { }
@@ -46,11 +47,11 @@ namespace StructureElement {
              * Indices into the unit.elements array.
              * Can use OrderedSet.forEach to iterate (or OrderedSet.size + OrderedSet.getAt)
              */
-            indices: OrderedSet<Index>
+            indices: OrderedSet<UnitIndex>
         }>
     }
 
-    export function Loci(elements: ArrayLike<{ unit: Unit, indices: OrderedSet<Index> }>): Loci {
+    export function Loci(elements: ArrayLike<{ unit: Unit, indices: OrderedSet<UnitIndex> }>): Loci {
         return { kind: 'element-loci', elements: elements as Loci['elements'] };
     }