diff --git a/crates/fj-core/src/algorithms/approx/curve/segment.rs b/crates/fj-core/src/algorithms/approx/curve/segment.rs
index 631a8923d..171a31ec0 100644
--- a/crates/fj-core/src/algorithms/approx/curve/segment.rs
+++ b/crates/fj-core/src/algorithms/approx/curve/segment.rs
@@ -99,13 +99,7 @@ impl CurveApproxSegment {
         );
         assert!(other.is_normalized(), "Can't merge non-normalized segment.");
 
-        let [self_min, self_max] = self.boundary.inner;
-        let [other_min, other_max] = other.boundary.inner;
-
-        let min = cmp::min(self_min, other_min);
-        let max = cmp::max(self_max, other_max);
-
-        self.boundary.inner = [min, max];
+        self.boundary = self.boundary.union(other.boundary);
 
         self.points.retain(|point| {
             // Only retain points that don't overlap with the other segment, or
diff --git a/crates/fj-core/src/geometry/boundary.rs b/crates/fj-core/src/geometry/boundary.rs
index d86a4eb84..7580e0b71 100644
--- a/crates/fj-core/src/geometry/boundary.rs
+++ b/crates/fj-core/src/geometry/boundary.rs
@@ -98,6 +98,32 @@ impl CurveBoundary<Point<1>> {
 
         Self { inner: [min, max] }
     }
+
+    /// Create the union of this boundary and another
+    ///
+    /// The result will be normalized.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the two boundaries don't overlap (touching counts as
+    /// overlapping).
+    pub fn union(self, other: Self) -> Self {
+        let self_ = self.normalize();
+        let other = other.normalize();
+
+        assert!(
+            self.overlaps(&other),
+            "Can't merge boundaries that don't at least touch"
+        );
+
+        let [self_min, self_max] = self_.inner;
+        let [other_min, other_max] = other.inner;
+
+        let min = cmp::min(self_min, other_min);
+        let max = cmp::max(self_max, other_max);
+
+        Self { inner: [min, max] }
+    }
 }
 
 impl<S, T: CurveBoundaryElement> From<[S; 2]> for CurveBoundary<T>