diff --git a/Eigen/src/Core/Matrix.h b/Eigen/src/Core/Matrix.h
index 4e24e84e2..5b95a0a5e 100644
--- a/Eigen/src/Core/Matrix.h
+++ b/Eigen/src/Core/Matrix.h
@@ -428,6 +428,11 @@ class Matrix
     explicit Matrix(const RotationBase<OtherDerived,ColsAtCompileTime>& r);
     template<typename OtherDerived>
     Matrix& operator=(const RotationBase<OtherDerived,ColsAtCompileTime>& r);
+
+    // allow to extend Matrix outside Eigen
+    #ifdef EIGEN_MATRIX_PLUGIN
+    #include EIGEN_MATRIX_PLUGIN
+    #endif
 };
 
 /** \defgroup matrixtypedefs Global matrix typedefs