diff --git a/include/GooseFEM/TyingsPeriodic.h b/include/GooseFEM/TyingsPeriodic.h
index d12ae4a..a77d624 100644
--- a/include/GooseFEM/TyingsPeriodic.h
+++ b/include/GooseFEM/TyingsPeriodic.h
@@ -1,226 +1,258 @@
 /**
 Tools to store and apply nodal/DOF tyings.
 
 \file TyingsPeriodic.h
 \copyright Copyright 2017. Tom de Geus. All rights reserved.
 \license This project is released under the GNU Public License (GPLv3).
 */
 
 #ifndef GOOSEFEM_TYINGSPERIODIC_H
 #define GOOSEFEM_TYINGSPERIODIC_H
 
 #include "config.h"
 
 #include <Eigen/Eigen>
 #include <Eigen/Sparse>
 
 namespace GooseFEM {
 
 /**
 Tools to store and apply nodal/DOF tyings.
 */
 namespace Tyings {
 
 /**
 Nodal tyings per periodic boundary conditions.
 The idea is that the displacement of all DOFs of a node are tied to another node
 and to the average displacement gradient.
 The latter is applied/measured using 'virtual' control nodes.
 
 Consider the DOF list \f$ u \f$ renumbered such that it is split up in
 independent and dependent DOFs as follows
 
 \f$ u = \begin{bmatrix} u_i \\ u_d \end{bmatrix}\f$
 
 whereby the independent DOFs are furthermore split up in unknown and prescribed nodes as follows
 
 \f$ u_i = \begin{bmatrix} u_u \\ u_p \end{bmatrix}\f$
 
 such that
 
 \f$ u = \begin{bmatrix} u_u \\ u_p \\ u_d \end{bmatrix}\f$
 
 \todo Document how the DOFs are tied to the control nodes, and what the has to do with the mean.
 */
 class Periodic {
 public:
 
     Periodic() = default;
 
     /**
     Constructor.
 
     \param coor Nodal coordinates [nnode, ndim].
     \param dofs DOF-numbers per node [nnode, ndim].
     \param control_dofs DOF-numbers per control node [ndim, ndim].
     \param nodal_tyings List of nodal tyings, see nodal_tyings(). [ntyings, 2].
     */
     Periodic(
         const xt::xtensor<double, 2>& coor,
         const xt::xtensor<size_t, 2>& dofs,
         const xt::xtensor<size_t, 2>& control_dofs,
         const xt::xtensor<size_t, 2>& nodal_tyings);
 
     /**
     Constructor.
 
     \param coor Nodal coordinates [nnode, ndim].
     \param dofs DOF-numbers per node [nnode, ndim].
     \param control_dofs DOF-numbers per control node [ndim, ndim].
     \param nodal_tyings List of nodal tyings, see nodal_tyings(). [ntyings, 2].
     \param iip List of prescribed DOF-numbers.
     */
     Periodic(
         const xt::xtensor<double, 2>& coor,
         const xt::xtensor<size_t, 2>& dofs,
         const xt::xtensor<size_t, 2>& control_dofs,
         const xt::xtensor<size_t, 2>& nodal_tyings,
         const xt::xtensor<size_t, 1>& iip);
 
     /**
     Number of dependent DOFs.
 
     \return unsigned int
     */
     size_t nnd() const;
 
     /**
     Number of independent DOFs.
 
     \return unsigned int
     */
     size_t nni() const;
 
     /**
     Number of independent unknown DOFs.
 
     \return unsigned int
     */
     size_t nnu() const;
 
     /**
     Number of independent prescribed DOFs.
 
     \return unsigned int
     */
     size_t nnp() const;
 
     /**
     Return the DOF-numbers per node, as used internally (after renumbering).
 
     \return [nnode, ndim].
     */
     xt::xtensor<size_t, 2> dofs() const;
 
     /**
     Return the DOF-numbers for each control node, as used internally (after renumbering).
 
     \return [ndim, ndim].
     */
     xt::xtensor<size_t, 2> control() const;
 
     /**
     Return the applied nodal tyings.
     Per tying (row) two node numbers are specified,
     according to the convention (independent, dependent).
 
     \return [ntyings, 2].
     */
     xt::xtensor<size_t, 2> nodal_tyings() const;
 
     /**
     Dependent DOFs.
 
     \return List of DOF numbers.
     */
     xt::xtensor<size_t, 1> iid() const;
 
     /**
     Independent DOFs.
 
     \return List of DOF numbers.
     */
     xt::xtensor<size_t, 1> iii() const;
 
     /**
     Independent unknown DOFs.
 
     \return List of DOF numbers.
     */
     xt::xtensor<size_t, 1> iiu() const;
 
     /**
     Independent prescribed DOFs.
 
     \return List of DOF numbers.
     */
     xt::xtensor<size_t, 1> iip() const;
 
     /**
     Return tying matrix such as to get the dependent DOFs \f$ u_d \f$ from
     the independent DOFs \f$ u_i \f$ as follows
 
     \f$ u_d = C_{di} u_i \f$
 
     Note that this can be further partitioned in
 
     \f$ u_d = C_{du} u_u + C_{dp} u_p \f$
 
     See Cdu() and Cdp().
 
     \return Sparse matrix.
     */
     Eigen::SparseMatrix<double> Cdi() const;
 
     /**
     Unknown part of the partitioned tying matrix, see Cdi().
 
     \return Sparse matrix.
     */
     Eigen::SparseMatrix<double> Cdu() const;
 
     /**
     Prescribed part of the partitioned tying matrix, see Cdi().
 
     \return Sparse matrix.
     */
     Eigen::SparseMatrix<double> Cdp() const;
 
 private:
     size_t m_nnu; ///< See nnu().
     size_t m_nnp; ///< See nnp().
     size_t m_nni; ///< See nni().
     size_t m_nnd; ///< See nnd().
     size_t m_ndim; ///< Number of dimensions.
     size_t m_nties; ///< Number of nodal ties.
     xt::xtensor<size_t, 2> m_dofs; ///< See dofs().
     xt::xtensor<size_t, 2> m_control; ///< See control().
     xt::xtensor<size_t, 2> m_tyings; ///< See nodal_tyings().
     xt::xtensor<double, 2> m_coor; ///< Nodal coordinates [nnode, ndim].
 };
 
+/**
+Add control nodes to an existing system.
+*/
 class Control {
 public:
-    // Constructors
+
     Control() = default;
+
+    /**
+    Constructor.
+
+    \param coor Nodal coordinates [nnode, ndim].
+    \param dofs DOF-numbers per node [nnode, ndim].
+    */
     Control(const xt::xtensor<double, 2>& coor, const xt::xtensor<size_t, 2>& dofs);
 
-    // Extract new lists
+    /**
+    Nodal coordinates, for the system with control nodes added to it.
+
+    \param [nnode + ndim, ndim], with nnode the number of nodes of the original system.
+    */
     xt::xtensor<double, 2> coor() const;
+
+    /**
+    DOF-numbers per node, for the system with control nodes added to it.
+
+    \param [nnode + ndim, ndim], with nnode the number of nodes of the original system.
+    */
     xt::xtensor<size_t, 2> dofs() const;
+
+    /**
+    DOF-numbers of each control node.
+
+    \param [ndim, ndim].
+    */
     xt::xtensor<size_t, 2> controlDofs() const;
+
+    /**
+    Node-numbers of the control nodes.
+
+    \param [ndim].
+    */
     xt::xtensor<size_t, 1> controlNodes() const;
 
 private:
-    xt::xtensor<double, 2> m_coor;
-    xt::xtensor<size_t, 2> m_dofs;
-    xt::xtensor<size_t, 2> m_control_dofs;
-    xt::xtensor<size_t, 1> m_control_nodes;
+    xt::xtensor<double, 2> m_coor; ///< See coor().
+    xt::xtensor<size_t, 2> m_dofs; ///< See dofs().
+    xt::xtensor<size_t, 2> m_control_dofs;  ///< See controlDofs().
+    xt::xtensor<size_t, 1> m_control_nodes; ///< See controlNodes().
 };
 
 } // namespace Tyings
 } // namespace GooseFEM
 
 #include "TyingsPeriodic.hpp"
 
 #endif