# Local Reference Frame

- Global Cartesian fixed system $X=\left(X\overrightarrow{i}+Y\overrightarrow{j}+Z\overrightarrow{k}\right)$
- Natural system $\left(\xi ,\eta ,\zeta \right)$ , covariant axes x,y
- Local systems (x, y, z) defined by an orthogonal set of unit base vectors ( ${t}_{1}$ , ${t}_{2}$ , $n$ ). $n$ is taken to be normal to the mid-surface coinciding with $\zeta $ , and ( ${t}_{1}$ , ${t}_{2}$ ) are taken in the tangent plane of the mid-surface.