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+
+****************
+Defining a Model
+****************
+
+To define your own model, you need a class for spin states, a class for spin transformations, a bond and site coupling, and a generator of transformations.
+
+Spin States
+===========
+
+.. cpp:class:: X_t
+
+   The spin type, which we have been denoting with :cpp:class:`X_t`, only needs to have a default constructor. If your spins can take only finitely many values, consider following the instructions in :ref:`finite_states` to significantly speed the algorithm.
+
+Transformations
+===============
+
+.. cpp:class:: R_t
+
+   The transformation type must have a default constructor, and the following member functions:
+
+   .. cpp:function:: X_t act(const X_t&)
+
+      The action of the transformation on a spin, returning the transformed spin.
+
+   .. cpp:function:: R_t act(const R_t&)
+
+      The action of the transformation on another transformation, returning the transformed transformation.
+
+   .. cpp:function:: X_t act_inverse(const X_t&)
+
+      The action of the inverse transformation on a spin, returning the transformed spin.
+
+   .. cpp:function:: R_t act_inverse(const R_t&)
+
+      The action of the inverse transformation on another transformation, returning the transformed transformation.
+
+Couplings
+=========
+
+When a :cpp:class:`system` object is initialized it needs to be given a bond and field coupling, to resemble the Hamiltonian
+
+.. math::
+         \mathcal H = -\sum_{\langle ij\rangle}Z(s_i,s_j)-\sum_iB(s_i)
+
+Note that building with certain compile flags will change the form that these coupling functions must take, as outlined in :ref:`compile`.
+
+Bond Coupling
+-------------
+
+.. cpp:function:: double Z(const X_t&, const X_t&)
+
+   A function that takes two spins and outputs the coupling between them. For traditional spin models, this is typically something like a dot product.
+
+Field Coupling
+--------------
+
+.. cpp:function:: double B(const X_t&)
+
+   A function that takes one spin and outputs the coupling between it and an external field.
+