  This paper describes in detail the construction of a local right-handed,
orthogonal curvilinear coordinate system whose axes are along the local
tangent, the local azimuth and the local normal of an analytically defined 3D
surface of rotation, whose shape mimics the shape of a plasma focus current
sheath. Expressions for various differential operators are derived in a
tutorial format for the benefit of young researchers and non-specialists.
Physical problems expressed in this coordinate system would benefit from the
natural symmetry properties of the plasma focus sheath. For example, the normal
component of current density is zero and the velocity has mainly the normal
component. This paper is meant to serve as a readily available reference in the
hope that it would be found useful.
