diff --git a/label_inscribed_600cell.js b/label_inscribed_600cell.js
index 600fc11..d2b3910 100644
--- a/label_inscribed_600cell.js
+++ b/label_inscribed_600cell.js
@@ -134,6 +134,21 @@ export function face_vertices(shape, f1, f2, f3) {
     return ns;
 }
 
+
+export function shared_neighbours(shape, nodes) {
+    // for f1/f2/f3 forming a triangular face, return the two vertices of the
+    // adjacent tetrahedra
+
+    let ns = shape.nodes.map((n) => n.id);
+    for( const n of nodes ) {
+        ns = neighbours_in_subset(shape, ns, n);
+    }
+
+    return ns;
+}
+
+
+
 export function layer_neighbours(cell600, layer) {
     console.log("Layer neighbours");
     for( const n of layer ) {