Every element has its own coordinate system, the element coordinate system, that determines the direction of orthotropic material properties, applied pressures, and results (such as stresses and strains) for that element. All element coordinate systems are right-handed orthogonal systems.
The default orientations for most elements' coordinate systems fit the following patterns:
Line elements usually have the element X-axis directed from their node I toward their node J.
Shell elements usually have the element X-axis similarly directed (from I toward J), the Z-axis normal to the shell surface (with the positive direction determined by the right-hand rule around the element from node I to J to K), and the Y-axis perpendicular to the X and Z axes.
For 2D and 3D solid elements, the element coordinate system is usually parallel to the global Cartesian system.
However, not all elements correspond to these patterns. See specific element descriptions in the Element Reference for the default element coordinate system orientation for such elements.
Many element types have key options (KEYOPTs, input at the time the element is defined (ET) or on the KEYOPT command) that allow you to change the default element coordinate system orientation. For area and volume elements, you can also change the orientation to align the element coordinate system with a previously defined local system by using the ESYS command.
If you specify both KEYOPTs and ESYS, the ESYS definition overrides. For some elements, you can define a further rotation, relative to the previous orientation, by entering an angle as a real constant.