Frenet trihedron

The classification of fields that we have identified can also be related to the concepts of curvature and torsion of a curve [6,26]. Indeed, time derivatives of unitary vectors s, t, b of a Frenet trihedron whose origins are set onto a point r in a curve, are expressed by the following relations ... 

These unit vectors are called the Frenet trihedron and provide a natural orthogonal coordinate system for space in the neighborhood of a point on a curve. 

As we travel along the curve, the Frenet trihedron rotates according to the Frenet-Serret equations... 

The vectors of the moving trihedron satisfy the Serret-Frenet equations... 

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