diff --git a/crates/doc/src/format.rs b/crates/doc/src/format.rs
index 2ee161ad..1e638d9c 100644
--- a/crates/doc/src/format.rs
+++ b/crates/doc/src/format.rs
@@ -13,13 +13,15 @@
 //! - 3: appearances per body, face and component with names and opacity
 //!   (`"appearances": {"bodies": {name: {name, color, opacity}}, "faces": […], "components": {…}}`);
 //!   before, `"appearances"` mapped body names to bare `[r, g, b]` colours.
+//! - 4: joint origins mate by default (B's z against A's, as Fusion does); `flip` now means
+//!   "don't mate". Older joints keep their placement: their `flip` is inverted.
 
 use serde_json::Value;
 
 use crate::{DocError, Document, Result};
 
 /// The format this build writes.
-pub const FORMAT: u32 = 3;
+pub const FORMAT: u32 = 4;
 
 pub fn format_string(n: u32) -> String {
     format!("solvecraft/{n}")
@@ -32,7 +34,20 @@ pub fn version_of(s: &str) -> Option<u32> {
 
 /// JSON-level upgrades: entry `i` turns format `i + 1` into `i + 2`.
 type JsonStep = fn(&mut Value) -> Result<()>;
-const JSON_STEPS: [JsonStep; 2] = [|_| Ok(()), v2_to_v3];
+const JSON_STEPS: [JsonStep; 3] = [|_| Ok(()), v2_to_v3, v3_to_v4];
+
+/// Joints placed with the old convention (aligned unless flipped) keep their placement.
+fn v3_to_v4(v: &mut Value) -> Result<()> {
+    if let Some(Value::Array(js)) = v.get_mut("assembly").and_then(|a| a.get_mut("joints")) {
+        for j in js.iter_mut() {
+            if let Some(o) = j.as_object_mut() {
+                let f = o.get("flip").and_then(Value::as_bool).unwrap_or(false);
+                o.insert("flip".into(), Value::Bool(!f));
+            }
+        }
+    }
+    Ok(())
+}
 
 /// Body colours `{name: [r, g, b]}` become named, opaque body appearances.
 fn v2_to_v3(v: &mut Value) -> Result<()> {
@@ -182,6 +197,12 @@ mod tests {
         old["appearances"] = serde_json::json!({"Body1": [200, 10, 20]});
         let d = read(&old.to_string()).unwrap();
         let l = d.appearances.bodies.get("Body1").unwrap();
+        // Format 3 joints (aligned unless flipped) keep their placement.
+        let mut j: Value = serde_json::from_str(&Document::new("A").to_json()).unwrap();
+        j["format"] = Value::from("solvecraft/3");
+        j["assembly"] = serde_json::json!({"joints": [{"id": 1, "name": "J", "kind": "rigid", "a": {"occurrence": 0, "snap": {"type": "point", "point": [0, 0, 0]}}, "b": {"occurrence": 0, "snap": {"type": "point", "point": [0, 0, 0]}}}]});
+        let dj = read(&j.to_string()).unwrap();
+        assert!(dj.assembly.joints[0].flip, "aligned before = not mated now");
         assert_eq!((l.color, l.opacity, l.name.as_str()), ([200, 10, 20], 1.0, "Custom"));
         let mut v: Value = serde_json::from_str(&Document::new("A").to_json()).unwrap();
         v["huge"] = Value::from(1e300);
diff --git a/crates/doc/src/joints.rs b/crates/doc/src/joints.rs
index 5c4a6eee..c3bfa8a6 100644
--- a/crates/doc/src/joints.rs
+++ b/crates/doc/src/joints.rs
@@ -122,7 +122,8 @@ pub struct Joint {
     /// Per value: (min, max) limits.
     #[serde(default)]
     pub limits: Vec<Option<(f64, f64)>>,
-    /// Turn B over (its z opposite to A's).
+    /// Don't mate: B's z along A's instead of against it (by default the origins mate, as two
+    /// faces meeting).
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub flip: bool,
     /// Distance from A's frame to B's along z, and a turn about it (radians).
@@ -404,7 +405,8 @@ pub fn solve(doc: &Document, st: &ModelState) -> Solution {
     let flip_m = |j: &Joint| {
         let mut m = rot_z(j.angle);
         m[3][2] = j.offset;
-        if j.flip { mat_mul(&m, &[[1.0, 0.0, 0.0, 0.0], [0.0, -1.0, 0.0, 0.0], [0.0, 0.0, -1.0, 0.0], [0.0, 0.0, 0.0, 1.0]]) } else { m }
+        // Origins mate (B's z against A's, as faces meet) unless flipped.
+        if !j.flip { mat_mul(&m, &[[1.0, 0.0, 0.0, 0.0], [0.0, -1.0, 0.0, 0.0], [0.0, 0.0, -1.0, 0.0], [0.0, 0.0, 0.0, 1.0]]) } else { m }
     };
     // B's world transform from A's (or A's from B's, going the other way).
     let through = |j: &Joint, from_a: bool, t: &Mat| -> Option<Mat> {
diff --git a/crates/engine/src/cmd/joints.rs b/crates/engine/src/cmd/joints.rs
index 292091fa..34c35427 100644
--- a/crates/engine/src/cmd/joints.rs
+++ b/crates/engine/src/cmd/joints.rs
@@ -303,7 +303,7 @@ fn as_built(s: &mut Session, p: &Value) -> Result<Value> {
         b: JointOrigin { occurrence: ob, snap: fb },
         values: vec![0.0; n],
         limits: limits_param(s, p, kind, cmd)?,
-        flip: false,
+        flip: true, // as built: the origins coincide as they are, not mated
         offset: 0.0,
         angle: 0.0,
         suppressed: false,
diff --git a/crates/engine/src/cmd/joints_tests.rs b/crates/engine/src/cmd/joints_tests.rs
index 2c6f3e10..7b091ed1 100644
--- a/crates/engine/src/cmd/joints_tests.rs
+++ b/crates/engine/src/cmd/joints_tests.rs
@@ -45,7 +45,7 @@ fn revolute_hinge_turns_about_the_knuckle() {
         &mut s,
         "JointAssembleCmdNew",
         json!({
-        "type": "revolute", "a": {"occurrence": "LeafA", "circle": [1, 37, 4]}, "b": {"occurrence": "LeafB", "circle": [1, 37, 4]}, "name": "Hinge"}),
+        "type": "revolute", "a": {"occurrence": "LeafA", "circle": [1, 37, 4]}, "b": {"occurrence": "LeafB", "circle": [1, 37, 4]}, "flip": true, "name": "Hinge"}),
     );
     assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
     // Same frames: B stays put at angle 0.
@@ -89,7 +89,7 @@ fn slider_limits_clamp() {
     let r = run(
         &mut s,
         "JointAssembleCmdNew",
-        json!({"type": "slider", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "flip": true, "limits": [[0, 10]], "name": "Rail"}),
+        json!({"type": "slider", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "limits": [[0, 10]], "name": "Rail"}),
     );
     assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
     // B now sits on A.
@@ -154,11 +154,7 @@ fn rigid_cycles_solve_and_conflicts_report() {
 #[test]
 fn motion_links_and_interference() {
     let mut s = two_cubes();
-    run(
-        &mut s,
-        "JointAssembleCmdNew",
-        json!({"type": "cylindrical", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "flip": true, "name": "Screw"}),
-    );
+    run(&mut s, "JointAssembleCmdNew", json!({"type": "cylindrical", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "name": "Screw"}));
     run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Screw", "values": [0, 0]}));
     // No overlap while B sits on A; then push B 4 mm into A.
     let i = run(&mut s, "InterferenceCheckCommand", json!({}));
@@ -174,7 +170,7 @@ fn motion_links_and_interference() {
     run(
         &mut s,
         "JointAssembleCmdNew",
-        json!({"type": "slider", "a": {"occurrence": "A", "face": [5, 5, 10]}, "b": {"occurrence": "D", "face": [65, 5, 0]}, "flip": true, "name": "Follower"}),
+        json!({"type": "slider", "a": {"occurrence": "A", "face": [5, 5, 10]}, "b": {"occurrence": "D", "face": [65, 5, 0]}, "name": "Follower"}),
     );
     run(&mut s, "FusionMotionRelationshipCommand", json!({"a": "Screw", "ia": 1, "b": "Follower", "ratio": 2}));
     run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Screw", "values": [0, 3]}));
@@ -186,3 +182,16 @@ fn motion_links_and_interference() {
     let l = run(&mut t, "joint.list", json!({}));
     assert_eq!(l["joints"].as_array().map(Vec::len), Some(2));
 }
+
+/// Two faces joined mate: B's bottom face lands on A's top face, B on top of A (not inside
+/// it); Flip turns B over instead.
+#[test]
+fn faces_mate_by_default() {
+    let mut s = two_cubes();
+    run(&mut s, "JointAssembleCmdNew", json!({"type": "rigid", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "name": "Stack"}));
+    // B's bottom centre (25, 5, 0) at A's top centre, B above it.
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 10.0)));
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 10.0)), Vec3::new(5.0, 5.0, 20.0)), "{:?}", place(&s, "B", Vec3::new(25.0, 5.0, 10.0)));
+    run(&mut s, "joint.edit", json!({"joint": "Stack", "flip": true}));
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 10.0)), Vec3::new(5.0, 5.0, 0.0)), "{:?}", place(&s, "B", Vec3::new(25.0, 5.0, 10.0)));
+}
diff --git a/crates/engine/src/cmd/motion_tests.rs b/crates/engine/src/cmd/motion_tests.rs
index a133632d..030ae964 100644
--- a/crates/engine/src/cmd/motion_tests.rs
+++ b/crates/engine/src/cmd/motion_tests.rs
@@ -32,7 +32,7 @@ fn rail() -> Session {
     run(&mut s, "component.activate", json!({"component": "root"}));
     let a = occ(&s, "A");
     run(&mut s, "occurrence.ground", json!({"occurrence": a, "grounded": true}));
-    run(&mut s, "JointAssembleCmdNew", json!({"type": "slider", "a": {"face": [10, 5, 5]}, "b": {"face": [20, 5, 5]}, "flip": true, "name": "Rail"}));
+    run(&mut s, "JointAssembleCmdNew", json!({"type": "slider", "a": {"face": [10, 5, 5]}, "b": {"face": [20, 5, 5]}, "name": "Rail"}));
     run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Rail", "value": 10}));
     assert!((b_x(&s) - 20.0).abs() < 1e-6, "{}", b_x(&s));
     s