diff --git a/crates/ui-egui/src/canvas/page_decor.rs b/crates/ui-egui/src/canvas/page_decor.rs
index 90119659b..0271b4bcd 100644
--- a/crates/ui-egui/src/canvas/page_decor.rs
+++ b/crates/ui-egui/src/canvas/page_decor.rs
@@ -82,8 +82,11 @@ fn draw_bleed_and_slug(painter: &egui::Painter, xf: &Xf, doc: &Document, slot: &
 fn draw_spine(painter: &egui::Painter, xf: &Xf, slot: &SpreadSlot, sp: &designcraft_doc::Spread, t: &Tokens) {
     let pt = |p: Point| xf.to_screen(slot.xf * p);
     for pair in sp.pages.windows(2) {
+        // Bound right to left, a spread lists its right page first: take the two edges that
+        // meet, whichever page comes first.
         let (a, b) = (pair[0].bounds(), pair[1].bounds());
-        let x = (a.x1 + b.x0) / 2.0;
+        let (left, right) = if a.x0 <= b.x0 { (a, b) } else { (b, a) };
+        let x = (left.x1 + right.x0) / 2.0;
         let (y0, y1) = (a.y0.min(b.y0), a.y1.max(b.y1));
         painter.line_segment([pt(Point::new(x, y0)), pt(Point::new(x, y1))], Stroke::new(hair(painter), t.text_dim));
     }
diff --git a/crates/ui-egui/src/canvas/tests.rs b/crates/ui-egui/src/canvas/tests.rs
index cdb73f3c2..584dd7ba5 100644
--- a/crates/ui-egui/src/canvas/tests.rs
+++ b/crates/ui-egui/src/canvas/tests.rs
@@ -78,6 +78,25 @@ fn a_spread_shows_a_spine_line_between_its_pages() {
     assert!(pairs > 0, "the document has a spread with two pages");
 }
 
+#[test]
+fn the_spine_sits_where_the_pages_of_a_right_to_left_spread_meet() {
+    // Bound right to left, the spread lists its right page first. With pages of different
+    // widths, the line must be at their shared edge, not halfway between the outer ones.
+    let mut app = DesignApp::new(designcraft_engine::Session::new(), crate::Services::default());
+    app.run("file.new", serde_json::json!({"pages": 3, "facingPages": true})).unwrap();
+    app.run("layout.documentSetup", serde_json::json!({"binding": "rightToLeft"})).unwrap();
+    let w = app.session.active().unwrap().doc.settings.page_width;
+    app.run("layout.pageSize", serde_json::json!({"pages": [2], "width": w * 0.6})).unwrap();
+    let shapes = canvas_shapes(&mut app);
+    let st = app.session.active().unwrap();
+    let layout = CanvasLayout::new(&st.doc, false);
+    let xf = Xf::new(app.canvas_rect.unwrap(), app.view().unwrap());
+    let (right, left) = (xf.rect(layout.page_rect(&st.doc, 1).unwrap()), xf.rect(layout.page_rect(&st.doc, 2).unwrap()));
+    assert!((left.max.x - right.min.x).abs() < 0.01, "page 3 is the left page, page 2 the right");
+    assert!((left.width() - right.width()).abs() > 1.0, "the pages differ in width");
+    assert_eq!(vertical_lines_at(&shapes, left.max.x, left), 1);
+}
+
 #[test]
 fn the_spine_line_hides_with_the_other_guides() {
     for (guides, mode) in [(false, ScreenMode::Normal), (true, ScreenMode::Preview), (true, ScreenMode::Presentation)] {