diff --git a/opm/core/pressure/mimetic/hybsys.h b/opm/core/pressure/mimetic/hybsys.h
index 34b534e5..44a7e7ce 100644
--- a/opm/core/pressure/mimetic/hybsys.h
+++ b/opm/core/pressure/mimetic/hybsys.h
@@ -397,6 +397,39 @@ hybsys_well_schur_comp_symm(int nc, const int *cwpos,
                             struct hybsys      *sys,
                             struct hybsys_well *wsys);
 
+/**
+ * Compute final (symmetric) Schur complement contributions to
+ * global system of simultaneous linear equations.
+ *
+ * This function forms the coefficient matrix
+ * \f[
+ * S_c = D^\mathsf{T}B_c^{-1}D - F_c^\mathsf{T}L_c^{-1}F_c
+ * \f]
+ * and similar right-hand side \f$r_c\f$ elemental contributions.
+ * These values must be subsequently assembled into the global system
+ * using function hybsys_global_assemble_cell() after imposing any
+ * applicable boundary conditions.
+ *
+ * This function overwrites the fields @c S and @c r of the hybrid system
+ * structure.
+ *
+ * @param[in]     c      Cell for which to compute local contributions.
+ * @param[in]     nconn  Number of connections (faces) of cell @c c.
+ * @param[in]     p1     Start address (into @c gpress) of the gravity
+ *                       contributions of cell @c c.
+ * @param[in]     p2     Start address (into @c Binv) of the inverse
+ *                       inner product of cell @c c.
+ * @param[in]     gpress Gravity contributions of all cells.  Must
+ *                       include effects of multiple phases if applicable.
+ * @param[in]     src    Explicit source terms for all cells.
+ * @param[in]     Binv   Inverse inner products for all cells.  Must
+ *                       include effects of multiple phases if applicable.
+ * @param[in,out] sys    Hybrid system management structure allocated
+ *                       using hybsys_allocate_symm() and initialised
+ *                       using hybsys_init() and/or filled using function
+ *                       hybsys_schur_comp_symm() and
+ *                       hybsys_well_schur_comp_symm() if applicable.
+ */
 void
 hybsys_cellcontrib_symm(int c, int nconn, int p1, int p2,
                         const double *gpress, const double *src,