= Outward conormal of a surface boundary
{title2=$u\,ds=d\mathbf r\times n$}
On the smooth boundary of an <orientable surface>, let $n$ be the chosen unit <normal vector> and $t$ the positively oriented unit <tangent vector> used by <Stokes theorem>. The outward conormal is $u=t\times n$. It is a <unit vector>, tangent to the surface and orthogonal to its boundary. This convention turns $(n\times v)\cdot d\mathbf r$ into $u\cdot v\,ds$ by cyclic invariance of the <scalar triple product>.
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