The orientation of the inclined surface can be defined by the angle $\mathrm{\theta }$ between the $\mathrm{x}$ axis and the normal $\mathrm{n}$ to section $\mathrm{b}-\mathrm{b}$. A positive angle $\mathrm{\theta }$ is defined as a counterclockwise rotation from the $\mathrm{x}$ axis to the $\mathrm{n}$ axis.

Note that the $\mathrm{n}-\mathrm{t}$ axes form a right-handed coordinate system