diff --git a/src/WaveBlocksND/DirectHomogeneousQuadrature.py b/src/WaveBlocksND/DirectHomogeneousQuadrature.py
index 9a3f3a7c..d46a5d13 100644
--- a/src/WaveBlocksND/DirectHomogeneousQuadrature.py
+++ b/src/WaveBlocksND/DirectHomogeneousQuadrature.py
@@ -13,20 +13,18 @@
 from scipy.linalg import sqrtm, inv #, svd, diagsvd
 
 from DirectQuadrature import DirectQuadrature
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["DirectHomogeneousQuadrature"]
 
 
-class DirectHomogeneousQuadrature(DirectQuadrature):
+class DirectHomogeneousQuadrature(DirectQuadrature, InnerProductCompatibility):
     r"""
     """
 
     def __init__(self, QR=None):
         # Pure convenience to allow setting of quadrature rule in constructor
-        if QR is not None:
-            self.set_qr(QR)
-        else:
-            self._QR = None
+        self.set_qr(QR)
 
 
     def __str__(self):
@@ -45,6 +43,10 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous",)
+
+
     def initialize_packet(self, packet):
         r"""Provide the wavepacket part of the inner product to evaluate.
         Since the quadrature is homogeneous the same wavepacket is used
diff --git a/src/WaveBlocksND/DirectInhomogeneousQuadrature.py b/src/WaveBlocksND/DirectInhomogeneousQuadrature.py
index 8b70aa28..812a81ab 100644
--- a/src/WaveBlocksND/DirectInhomogeneousQuadrature.py
+++ b/src/WaveBlocksND/DirectInhomogeneousQuadrature.py
@@ -14,20 +14,18 @@
 from scipy.linalg import sqrtm, inv, det
 
 from DirectQuadrature import DirectQuadrature
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["DirectInhomogeneousQuadrature"]
 
 
-class DirectInhomogeneousQuadrature(DirectQuadrature):
+class DirectInhomogeneousQuadrature(DirectQuadrature, InnerProductCompatibility):
     r"""
     """
 
     def __init__(self, QR=None):
         # Pure convenience to allow setting of quadrature rule in constructor
-        if QR is not None:
-            self.set_qr(QR)
-        else:
-            self._QR = None
+        self.set_qr(QR)
 
 
     def __str__(self):
@@ -46,6 +44,10 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous", "inhomogeneous",)
+
+
     def initialize_packet(self, pacbra, packet=None):
         r"""Provide the wavepacket parts of the inner product to evaluate.
         Since the quadrature is inhomogeneous different wavepackets can be
diff --git a/src/WaveBlocksND/HomogeneousInnerProduct.py b/src/WaveBlocksND/HomogeneousInnerProduct.py
index dc3ef9c6..6a5ccca8 100644
--- a/src/WaveBlocksND/HomogeneousInnerProduct.py
+++ b/src/WaveBlocksND/HomogeneousInnerProduct.py
@@ -12,11 +12,12 @@
 from numpy import zeros, complexfloating, sum, cumsum
 
 from InnerProduct import InnerProduct
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["HomogeneousInnerProduct"]
 
 
-class HomogeneousInnerProduct(InnerProduct):
+class HomogeneousInnerProduct(InnerProduct, InnerProductCompatibility):
     r"""
     """
 
@@ -48,6 +49,14 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous",)
+
+
+    def require_kind(self):
+        return ("homogeneous",)
+
+
     def quadrature(self, packet, operator=None, summed=False, component=None, diag_component=None, diagonal=False, eval_at_once=False):
         r"""Delegates the evaluation of :math:`\langle\Psi|f|\Psi\rangle` for a general
         function :math:`f(x)` with :math:`x \in \mathbb{R}^D`.
diff --git a/src/WaveBlocksND/HomogeneousInnerProductLCWP.py b/src/WaveBlocksND/HomogeneousInnerProductLCWP.py
index 3ca00ba7..7dc375f3 100644
--- a/src/WaveBlocksND/HomogeneousInnerProductLCWP.py
+++ b/src/WaveBlocksND/HomogeneousInnerProductLCWP.py
@@ -13,11 +13,12 @@
 from numpy import zeros, complexfloating, conjugate, transpose, dot, cumsum, sum, reshape, array, repeat
 
 from InnerProduct import InnerProduct
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["HomogeneousInnerProductLCWP"]
 
 
-class HomogeneousInnerProductLCWP(InnerProduct):
+class HomogeneousInnerProductLCWP(InnerProduct, InnerProductCompatibility):
 
     def __init__(self, delegate=None, oracle=None):
         r"""
@@ -55,6 +56,14 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous",)
+
+
+    def require_kind(self):
+        return ("homogeneous",)
+
+
     def get_oracle(self):
         r"""Return the sparsity oracle in use or ``None``.
         """
diff --git a/src/WaveBlocksND/InhomogeneousInnerProduct.py b/src/WaveBlocksND/InhomogeneousInnerProduct.py
index 679d634b..fe8fda53 100644
--- a/src/WaveBlocksND/InhomogeneousInnerProduct.py
+++ b/src/WaveBlocksND/InhomogeneousInnerProduct.py
@@ -12,11 +12,12 @@
 from numpy import zeros, complexfloating, sum, cumsum
 
 from InnerProduct import InnerProduct
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["InhomogeneousInnerProduct"]
 
 
-class InhomogeneousInnerProduct(InnerProduct):
+class InhomogeneousInnerProduct(InnerProduct, InnerProductCompatibility):
     r"""
     """
 
@@ -48,6 +49,14 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous", "inhomogeneous",)
+
+
+    def require_kind(self):
+        return ("inhomogeneous",)
+
+
     def quadrature(self, pacbra, packet=None, operator=None, summed=False, component=None, diag_component=None, diagonal=False, eval_at_once=False):
         r"""Delegates the evaluation of :math:`\langle\Psi|f|\Psi^\prime\rangle` for a general
         function :math:`f(x)` with :math:`x \in \mathbb{R}^D`.
diff --git a/src/WaveBlocksND/InhomogeneousInnerProductLCWP.py b/src/WaveBlocksND/InhomogeneousInnerProductLCWP.py
index f6a98eac..b06b544d 100644
--- a/src/WaveBlocksND/InhomogeneousInnerProductLCWP.py
+++ b/src/WaveBlocksND/InhomogeneousInnerProductLCWP.py
@@ -13,11 +13,12 @@
 from numpy import zeros, complexfloating, conjugate, transpose, dot, sum, cumsum, array, repeat, reshape
 
 from InnerProduct import InnerProduct
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["InhomogeneousInnerProductLCWP"]
 
 
-class InhomogeneousInnerProductLCWP(InnerProduct):
+class InhomogeneousInnerProductLCWP(InnerProduct, InnerProductCompatibility):
 
     def __init__(self, delegate=None, oracle=None):
         r"""
@@ -55,6 +56,14 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous", "inhomogeneous",)
+
+
+    def require_kind(self):
+        return ("inhomogeneous",)
+
+
     def get_oracle(self):
         r"""Return the sparsity oracle in use or ``None``.
         """
diff --git a/src/WaveBlocksND/InnerProduct.py b/src/WaveBlocksND/InnerProduct.py
index bac21b23..7e621fc2 100644
--- a/src/WaveBlocksND/InnerProduct.py
+++ b/src/WaveBlocksND/InnerProduct.py
@@ -11,10 +11,12 @@
 @license: Modified BSD License
 """
 
+from InnerProductCompatibility import InnerProductCompatibility
+
 __all__ = ["InnerProduct", "InnerProductException"]
 
 
-class InnerProduct(object):
+class InnerProduct(InnerProductCompatibility):
     r"""This class is an abstract interface to inner products in general.
     """
 
@@ -45,7 +47,11 @@ def set_delegate(self, delegate):
         :param delegate: The new :py:class:`Quadrature` instance.
         """
         # TODO: Allow a list of quads, one quad for each component of Psi
-        self._delegate = delegate
+        if delegate is not None:
+            if self.compatible(self, delegate):
+                self._delegate = delegate
+        else:
+            self._delegate = delegate
 
 
     def get_delegate(self):
diff --git a/src/WaveBlocksND/InnerProductCompatibility.py b/src/WaveBlocksND/InnerProductCompatibility.py
new file mode 100644
index 00000000..e76bcce8
--- /dev/null
+++ b/src/WaveBlocksND/InnerProductCompatibility.py
@@ -0,0 +1,32 @@
+"""The WaveBlocks Project
+
+This class abstracts compatibility conditions on nested inner products.
+
+@author: R. Bourquin
+@copyright: Copyright (C) 2013 R. Bourquin
+@license: Modified BSD License
+"""
+
+__all__ = ["InnerProductCompatibility"]
+
+
+class InnerProductCompatibility(object):
+    r"""This class abstracts compatibility conditions on nested inner products.
+    """
+
+    def get_kind(self):
+        return None
+
+    def require_kind(self):
+        return None
+
+    def compatible(self, ipouter, ipinner):
+        r"""
+        """
+        inner = set(ipinner.get_kind())
+        outer = set(ipouter.require_kind())
+
+        if not len(outer.intersection(inner)) == 0:
+            return True
+        else:
+            raise ValueError("Can not nest inner product with kind "+str(inner)+" into inner product which requires "+str(outer))
diff --git a/src/WaveBlocksND/NSDInhomogeneous.py b/src/WaveBlocksND/NSDInhomogeneous.py
index e9f4c836..c77f3d29 100644
--- a/src/WaveBlocksND/NSDInhomogeneous.py
+++ b/src/WaveBlocksND/NSDInhomogeneous.py
@@ -15,11 +15,12 @@
 from scipy.linalg import inv, schur, det, sqrtm
 
 from Quadrature import Quadrature
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["NSDInhomogeneous"]
 
 
-class NSDInhomogeneous(Quadrature):
+class NSDInhomogeneous(Quadrature, InnerProductCompatibility):
     r"""
     """
 
@@ -30,10 +31,7 @@ def __init__(self, QR=None):
         :param QR: Typically one uses an instance of :py:class:`GaussHermiteOriginalQR`.
         """
         # Pure convenience to allow setting of quadrature rule in constructor
-        if QR is not None:
-            self.set_qr(QR)
-        else:
-            self._QR = None
+        self.set_qr(QR)
 
 
     def __str__(self):
@@ -52,6 +50,10 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous", "inhomogeneous")
+
+
     def initialize_packet(self, pacbra, packet=None):
         r"""Provide the wavepacket parts of the inner product to evaluate.
         Since the quadrature is inhomogeneous, different wavepackets can be
diff --git a/src/WaveBlocksND/SymbolicIntegral.py b/src/WaveBlocksND/SymbolicIntegral.py
index bc5c1f79..6b6a3dfd 100644
--- a/src/WaveBlocksND/SymbolicIntegral.py
+++ b/src/WaveBlocksND/SymbolicIntegral.py
@@ -17,11 +17,12 @@
 
 from InnerProduct import InnerProductException
 from Quadrature import Quadrature
+from InnerProductCompatibility import InnerProductCompatibility
 
 __all__ = ["SymbolicIntegral"]
 
 
-class SymbolicIntegral(Quadrature):
+class SymbolicIntegral(Quadrature, InnerProductCompatibility):
     r"""
     """
 
@@ -53,6 +54,10 @@ def get_description(self):
         return d
 
 
+    def get_kind(self):
+        return ("homogeneous", "inhomogeneous")
+
+
     def initialize_packet(self, pacbra, packet=None):
         r"""Provide the wavepacket parts of the inner product to evaluate.
         Since the formula is for the inhomogeneous case explicitly, different
diff --git a/src/WaveBlocksND/__init__.py b/src/WaveBlocksND/__init__.py
index 06290822..ddfd56f7 100644
--- a/src/WaveBlocksND/__init__.py
+++ b/src/WaveBlocksND/__init__.py
@@ -81,6 +81,8 @@
 from HomogeneousInnerProductLCWP import HomogeneousInnerProductLCWP
 from InhomogeneousInnerProductLCWP import InhomogeneousInnerProductLCWP
 
+#from InnerProductCompatibility import InnerProductCompatibility
+
 from Quadrature import Quadrature
 from DirectQuadrature import DirectQuadrature
 from DirectHomogeneousQuadrature import DirectHomogeneousQuadrature