diff --git a/crates/engine/src/cmd/joints.rs b/crates/engine/src/cmd/joints.rs
index 4b8fdbfe..1a346467 100644
--- a/crates/engine/src/cmd/joints.rs
+++ b/crates/engine/src/cmd/joints.rs
@@ -305,8 +305,29 @@ fn origin_param(s: &Session, v: Option<&Value>, cmd: &str) -> Result<JointOrigin
         }
     };
     let o = JointOrigin { occurrence, snap };
-    joints::resolve_origin(&s.doc, &s.model.state(), &o).map_err(|e| bad(cmd, e.to_string()))?;
-    Ok(o)
+    match joints::resolve_origin(&s.doc, &s.model.state(), &o) {
+        Ok(_) => Ok(o),
+        // A circle given by its centre, where no face lies (#393).
+        Err(e) => match (what == "circle").then(|| circle_centre(s, occurrence, local(world))).flatten() {
+            Some(snap) => Ok(JointOrigin { occurrence, snap }),
+            None => Err(bad(cmd, e.to_string())),
+        },
+    }
+}
+
+/// The round end of a cylindrical face of the occurrence's bodies centred at `pick`: a point on
+/// its axis there, pointing out of that end (as picking its circular edge gives).
+fn circle_centre(s: &Session, occurrence: u64, pick: Vec3) -> Option<Snap> {
+    let component = if occurrence == 0 { 0 } else { s.doc.occurrences.iter().find(|o| o.id == occurrence)?.component };
+    let st = s.model.state();
+    let c = st
+        .bodies
+        .iter()
+        .filter(|b| s.doc.model_body_component(b) == component)
+        .find_map(|b| solidcraft_doc::kernel::cylinder_end_at(&b.body, pick))?;
+    let along = (pick - c.axis_point).dot(c.axis);
+    let (at, dir) = if (along - c.start).abs() <= (c.end - along).abs() { (c.start, -1.0) } else { (c.end, 1.0) };
+    Some(Snap::Point { point: c.axis_point + c.axis * at, z: Some(c.axis * dir) })
 }
 
 pub(crate) fn joint_id(s: &Session, v: Option<&Value>, cmd: &str) -> Result<u64> {
@@ -699,7 +720,10 @@ fn size_param(s: &Session, p: &Value, key: &str, cmd: &str) -> Result<Size> {
     match p.get(key) {
         Some(Value::Number(n)) => n.as_f64().map(Size::Length).ok_or_else(|| bad(cmd, format!("`{key}` must be a finite length"))),
         Some(Value::String(t)) if t.len() <= 1000 => Ok(Size::Expr(t.trim().to_string())),
-        Some(o @ Value::Object(m)) if m.contains_key("circle") => Ok(Size::Circle(origin_param(s, Some(o), cmd)?)),
+        Some(o @ Value::Object(m)) if m.contains_key("circle") => match origin_param(s, Some(o), cmd)? {
+            o @ JointOrigin { snap: Snap::CircleCenter { .. }, .. } => Ok(Size::Circle(o)),
+            _ => Err(bad(cmd, format!("`{key}`: give a point on the round face, not its centre"))),
+        },
         _ => Err(bad(cmd, format!("`{key}` must be a length (mm or an expression) or a round face ({{circle: [x, y, z]}})"))),
     }
 }
diff --git a/crates/engine/src/cmd/joints_tests.rs b/crates/engine/src/cmd/joints_tests.rs
index e86571f8..2abeadb5 100644
--- a/crates/engine/src/cmd/joints_tests.rs
+++ b/crates/engine/src/cmd/joints_tests.rs
@@ -34,6 +34,40 @@ fn leaf(s: &mut Session, name: &str) {
     run(s, "solid.cylinder", json!({"base": [0, 33, 4], "axis": [1, 0, 0], "radius": 4, "height": 40, "operation": "new"}));
 }
 
+#[test]
+fn circle_given_by_its_centre_takes_that_round_end() {
+    // #393: `circle` at the centre of a round end, as the parameters document, was refused with
+    // "no cylindrical face there" (no face lies at a centre).
+    let mut s = Session::default();
+    run(&mut s, "component.create", json!({"name": "Plate"}));
+    run(&mut s, "solid.box", json!({"length": 30, "width": 20, "height": 6, "corner": [-15, -10, 0]}));
+    run(&mut s, "component.activate", json!({"component": "root"}));
+    run(&mut s, "component.create", json!({"name": "Knob"}));
+    run(&mut s, "solid.cylinder", json!({"base": [0, 0, 6], "axis": [0, 0, 1], "radius": 10, "height": 10, "operation": "new"}));
+    let r = run(
+        &mut s,
+        "joint.create",
+        json!({"type": "cylindrical", "a": {"occurrence": "Plate", "face": [0, 0, 6]}, "b": {"occurrence": "Knob", "circle": [0, 0, 6]}, "name": "J"}),
+    );
+    assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
+    // The knob's bottom circle already sits on the plate's top: its axis stays put (it may turn
+    // about it, to line up the two frames' x).
+    for z in [6.0, 16.0] {
+        assert!(near(place(&s, "Knob", Vec3::new(0.0, 0.0, z)), Vec3::new(0.0, 0.0, z)), "{:?}", place(&s, "Knob", Vec3::new(0.0, 0.0, z)));
+    }
+    // The top end's centre: its axis points up, out of the knob.
+    run(&mut s, "joint.origin", json!({"occurrence": "Knob", "circle": [0, 0, 16], "name": "Top"}));
+    let top = s.doc.assembly.origins.iter().find(|o| o.name == "Top").map(|o| o.origin.snap.clone());
+    assert!(
+        matches!(top, Some(solidcraft_doc::joints::Snap::Point { point, z: Some(z) }) if near(point, Vec3::new(0.0, 0.0, 16.0)) && near(z, Vec3::Z)),
+        "{top:?}"
+    );
+    // Off the axis is still refused, and so is a coupling size (it follows a face).
+    assert!(s.execute("joint.origin", &json!({"occurrence": "Knob", "circle": [3, 0, 6]})).is_err());
+    let e = s.execute("joint.screw", &json!({"joint": "J", "pitch": {"occurrence": "Knob", "circle": [0, 0, 6]}})).unwrap_err();
+    assert!(e.to_string().contains("not its centre"), "{e}");
+}
+
 #[test]
 fn revolute_hinge_turns_about_the_knuckle() {
     let mut s = Session::default();
diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index daf37d7b..b4907752 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -181,7 +181,7 @@ pub use surfaces::{
     sweep_sheet_points, sweep_sheet_points_with, sweep_sheet_with, thicken, trim, unstitch,
 };
 pub use sweep_path::{SweepControl, SweepOrientation, sweep_path, sweep_path_with};
-pub use topo::{CylinderFace, cylinder_face_at};
+pub use topo::{CylinderFace, cylinder_end_at, cylinder_face_at};
 pub use topo::{TopoCounts, merged_topology, seam_flags};
 pub use tspline::tspline_sheet;
 
diff --git a/crates/kernel/src/topo.rs b/crates/kernel/src/topo.rs
index 32c5c6aa..60be2b42 100644
--- a/crates/kernel/src/topo.rs
+++ b/crates/kernel/src/topo.rs
@@ -663,6 +663,32 @@ pub fn cylinder_face_at(b: &Body, p: Vec3) -> Option<CylinderFace> {
     let tol = (size * 1e-4).max(1e-6);
     let surfs = classify_faces(b, &mesh, nf, tol);
     let Surf::Cylinder { axis, p: ap, r } = *surfs.get(face as usize)? else { return None };
+    Some(cylinder_extent(&mesh, face, axis, ap, r))
+}
+
+/// The cylindrical face one of whose end circles is centred at `p` (a circle given by its
+/// centre, where no face lies), the nearest when several are.
+pub fn cylinder_end_at(b: &Body, p: Vec3) -> Option<CylinderFace> {
+    let size = b.size();
+    let tol = (size * 1e-3).max(1e-3);
+    let mesh = b.tessellate(tol).ok()?;
+    let surfs = classify_faces(b, &mesh, b.face_count(), (size * 1e-4).max(1e-6));
+    let mut best: Option<(f64, CylinderFace)> = None;
+    for (face, s) in surfs.iter().enumerate() {
+        let Surf::Cylinder { axis, p: ap, r } = *s else { continue };
+        let c = cylinder_extent(&mesh, u32::try_from(face).ok()?, axis, ap, r);
+        let v = p - c.axis_point;
+        let along = v.dot(c.axis);
+        let off = (v - c.axis * along).len() + (along - c.start).abs().min((c.end - along).abs());
+        if off <= tol && best.is_none_or(|(bo, _)| off < bo) {
+            best = Some((off, c));
+        }
+    }
+    best.map(|(_, c)| c)
+}
+
+/// The extent along its axis, and the side, of a cylindrical face of a tessellated body.
+fn cylinder_extent(mesh: &Mesh, face: u32, axis: Vec3, ap: Vec3, r: f64) -> CylinderFace {
     let mut lo = f64::INFINITY;
     let mut hi = f64::NEG_INFINITY;
     let mut inward = 0.0;
@@ -679,5 +705,5 @@ pub fn cylinder_face_at(b: &Body, p: Vec3) -> Option<CylinderFace> {
             inward += n.dot(radial);
         }
     }
-    Some(CylinderFace { axis_point: ap, axis, radius: r, start: lo, end: hi, internal: inward < 0.0 })
+    CylinderFace { axis_point: ap, axis, radius: r, start: lo, end: hi, internal: inward < 0.0 }
 }