# Monoclinic anisotropy

${\displaystyle \{c_{\alpha \beta }\}={\begin{pmatrix}c_{11}&c_{12}&c_{13}&0&0&c_{16}\\c_{12}&c_{22}&c_{23}&0&0&c_{26}\\c_{13}&c_{23}&c_{33}&0&0&c_{36}\\0&0&0&c_{44}&0&0\\0&0&0&0&c_{55}&0\\c_{16}&c_{26}&c_{36}&0&0&c_{66}\\\end{pmatrix}}}$
Assuming that the one symmetry plane is horizontal, and that the survey coordinate system has one axis vertical, it is common that the horizontal axes of the survey coordinate system are not aligned with those of the principal coordinate system. In this case, the stiffness matrix (referred to the survey coordinate system) has the same form as above, but with ${\displaystyle c_{45}=c_{54}}$ non-zero, so that effectively there are 13 elements to determine. Dealing with such systems is beyond the limit of feasibility in 2020; the same is true for all lower symmetries.[1]