Besides conventional rectangular lattices, EM.Cube's [[Planar Module]] can also handle complex non-rectangular periodic lattices. For example, many frequency selective surfaces have skewed grids. In order to simulate skewed-grid periodic structures, the definition of the grid has to be generalized. A periodic structure is a repetition of a basic structure (unit cell) at pre-determined locations. Let these locations be described by (x<sub>mn</sub>, y<sub>mn</sub>), where m and n are integers ranging from -8 to 8. For a general skewed grid, x<sub>mn</sub> and y<sub>mn</sub> can be described by:
:<math>\begin{align}& x_{mn} = m\Delta x + n \Delta x'</math>\\:<math>& y_{mn} = m\Delta y + n \Delta y'\end{align}</math>
where <math>\Delta x</math> is the primary offset in the X direction (X Spacing) controlled by index m and <math>\Delta x'</math> is the secondary offset in the X direction (X Offset) controlled by index n. The meanings of <math>\Delta y</math> (Y Spacing) and <math>\Delta y'</math> (Y Offset) are similar with the roles of indices m and n interchanged. To illustrate how to use this definition, consider an example of an equilateral triangular grid with side length L as shown in the figure below.