diff --git a/src/lib.rs b/src/lib.rs
index b5088b4e..ba55262a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -100,6 +100,7 @@ mod mindist;
 pub mod offset;
 mod param_curve;
 mod point;
+mod primitive_shape;
 mod quadbez;
 mod quadspline;
 mod rect;
@@ -125,6 +126,7 @@ pub use crate::insets::*;
 pub use crate::line::*;
 pub use crate::param_curve::*;
 pub use crate::point::*;
+pub use crate::primitive_shape::*;
 pub use crate::quadbez::*;
 pub use crate::quadspline::*;
 pub use crate::rect::*;
diff --git a/src/primitive_shape.rs b/src/primitive_shape.rs
new file mode 100644
index 00000000..d4619613
--- /dev/null
+++ b/src/primitive_shape.rs
@@ -0,0 +1,177 @@
+// Copyright 2024 the Kurbo Authors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+#![allow(unused_qualifications)]
+
+use crate::{Point, Rect, Shape};
+use alloc::boxed::Box;
+
+mod _never_shape {
+    use super::*;
+    use crate::PathEl;
+    /// An uninhabited type that implements shape.
+    #[derive(Debug, Clone, Copy, PartialEq)]
+    #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+    #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+    pub enum NeverShape {}
+    impl Shape for NeverShape {
+        type PathElementsIter<'a> = core::iter::Empty<PathEl>;
+        fn path_elements(&self, _: f64) -> Self::PathElementsIter<'_> {
+            unreachable!()
+        }
+        fn area(&self) -> f64 {
+            unreachable!()
+        }
+        fn perimeter(&self, _: f64) -> f64 {
+            unreachable!()
+        }
+        fn winding(&self, _: Point) -> i32 {
+            unreachable!()
+        }
+        fn bounding_box(&self) -> Rect {
+            unreachable!()
+        }
+    }
+}
+
+/// Because the `Shape` trait is not dyn safe, it can be difficult to store
+/// Collections of `Shape` items in hetereogenuous collections.
+///
+/// It allows an external `Shape` impl to be provided as an extension point
+/// for shape impls provided by external crates.  This defaults to an
+/// uninhabited type.
+#[derive(Clone, Debug, PartialEq)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[non_exhaustive]
+pub enum ConcreteShape<External = _never_shape::NeverShape>
+where
+    External: Shape,
+{
+    /// Corresponds to a `PathSeg`
+    PathSeg(crate::PathSeg),
+    /// Corresponds to an `Arc`
+    Arc(crate::Arc),
+    /// Corresponds to a `BezPath`
+    BezPath(crate::BezPath),
+    /// Corresponds to a `Circle`
+    Circle(crate::Circle),
+    /// Corresponds to a `CircleSegment`
+    CircleSegment(crate::CircleSegment),
+    /// Corresponds to a `CubicBez`
+    CubicBez(crate::CubicBez),
+    /// Corresponds to an `Ellipse`
+    Ellipse(crate::Ellipse),
+    /// Corresponds to a `Line`
+    Line(crate::Line),
+    /// Corresponds to a `QuadBez`
+    QuadBez(crate::QuadBez),
+    /// Corresponds to a `Rect`
+    Rect(Rect),
+    /// Corresponds to a `RoundedRect`
+    RoundedRect(crate::RoundedRect),
+    /// A type implementing shape that may be defined by an external crate.
+    External(External),
+}
+
+macro_rules! from_shape {
+    ($it: ident) => {
+        impl From<crate::$it> for ConcreteShape {
+            fn from(it: crate::$it) -> Self {
+                Self::$it(it)
+            }
+        }
+    };
+}
+
+from_shape!(PathSeg);
+from_shape!(Arc);
+from_shape!(BezPath);
+from_shape!(Circle);
+from_shape!(CircleSegment);
+from_shape!(CubicBez);
+from_shape!(Ellipse);
+from_shape!(Line);
+from_shape!(QuadBez);
+from_shape!(Rect);
+from_shape!(RoundedRect);
+
+impl ConcreteShape {
+    /// Builds a static shape from an external shape implementation.
+    ///
+    /// For a kurbo provided shape implementation, you would normally use the `from` impl instead.
+    pub fn from_external_shape<External>(shape: External) -> ConcreteShape<External>
+    where
+        External: Shape,
+    {
+        ConcreteShape::External(shape)
+    }
+}
+
+macro_rules! match_shape {
+    ($x:ident, $it:ident, $e: expr) => {
+        match $x {
+            ConcreteShape::PathSeg($it) => $e,
+            ConcreteShape::Arc($it) => $e,
+            ConcreteShape::BezPath($it) => $e,
+            ConcreteShape::Circle($it) => $e,
+            ConcreteShape::CircleSegment($it) => $e,
+            ConcreteShape::CubicBez($it) => $e,
+            ConcreteShape::Ellipse($it) => $e,
+            ConcreteShape::Line($it) => $e,
+            ConcreteShape::QuadBez($it) => $e,
+            ConcreteShape::Rect($it) => $e,
+            ConcreteShape::RoundedRect($it) => $e,
+            ConcreteShape::External($it) => $e,
+        }
+    };
+}
+
+impl<External> Shape for ConcreteShape<External>
+where
+    External: Shape,
+{
+    type PathElementsIter<'iter> = Box<dyn Iterator<Item = crate::PathEl> + 'iter> where External: 'iter;
+    fn path_elements(&self, tol: f64) -> Box<dyn Iterator<Item = crate::PathEl> + '_> {
+        match_shape!(self, it, Box::new(it.path_elements(tol)))
+    }
+
+    fn perimeter(&self, acc: f64) -> f64 {
+        match_shape!(self, it, it.perimeter(acc))
+    }
+
+    fn area(&self) -> f64 {
+        match_shape!(self, it, it.area())
+    }
+
+    fn winding(&self, pt: Point) -> i32 {
+        match_shape!(self, it, it.winding(pt))
+    }
+
+    fn bounding_box(&self) -> Rect {
+        match_shape!(self, it, it.bounding_box())
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn test_collection() {
+        let r = crate::Rect::from_origin_size((0.0, 0.0), (1.0, 1.0));
+        let l = crate::Line::new((0.0, 0.0), (0.5, 0.5));
+        let shapes: Vec<ConcreteShape> = vec![r.into(), l.into()];
+        assert_eq!(
+            shapes,
+            vec![ConcreteShape::Rect(r), ConcreteShape::Line(l),]
+        );
+    }
+    #[test]
+    fn test_external() {
+        let l = crate::Line::new((0.0, 0.0), (0.5, 0.5));
+        assert_eq!(
+            ConcreteShape::from_external_shape(l),
+            ConcreteShape::External(l)
+        );
+    }
+}