# Triangles Using a Characterised Surface Definition

This option defines a characterised surface.

The principal direction in each layer is defined by the angle $\alpha$ (see Angle in the dialog image) relative to the projection of the vector $\zeta$ (Reference direction group) onto the plane of the triangle (in the DI card). Here $\alpha$ is measured in the mathematically positive sense with respect to the normal vector of the triangle.

In this case the card line is followed by an additional line for each layer. The medium properties in the principle direction is different from those in the orthogonal direction which lies in the plane of the triangle and orthogonal to the principal direction.

## Parameters

Reference direction
The X, Y and Z components of the vector $\zeta$ (used to define the principal direction, see image on the dialog).
Characterised surface label
Label of the characterised surface medium as specified in the DI card.