NOTE A vector line element is preferably designated by $e_{\text{t}}\mathrm{d}s$ or by $\mathrm{d}r$, or sometimes by $t\mathrm{d}s$, where $e_{\text{t}}=t$ is a unit vector tangential to the curve, $\mathrm{d}s$ is a scalar line element, $\mathrm{d}r$ is the differential of the position vector r describing the curve with respect to an origin point.