A guide on tensors is proposed for undergraduate students in physics or engineering that ties directly to vector calculus in orthonormal coordinate systems. We show that once orthonormality is relaxed, a dual basis, together with the contravariant and covariant components, naturally emerges. Manipulating these components requires some skill that can be acquired more easily and quickly once a new notation is adopted. This notation distinguishes multi-component quantities in different coordinate systems by a differentiating sign on the index labelling the component rather than on the label of the quantity itself. This tiny stratagem, together with simple rules openly stated at the beginning of this guide, allows an almost automatic, easy-to-pursue procedure for what is otherwise a cumbersome algebra. By the end of the paper, the reader will be skillful enough to tackle many applications involving tensors of any rank in any coordinate system, without index-manipulation obstacles standing in the way.

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18.
We are also assuming that the readers have been exposed to Cramer's rule for solving linear algebraic equations, i.e., that they have some familiarity with the properties of determinants and the inversion of matrices.
19.
For completeness, we should add that multiplication by 1 leaves a vector unchanged.
20.
For vectors defined in the complex field, the scalar product would not be commutative.
21.
In the context of crystallography, the lattice space and its basis are called the direct space and direct basis, whereas the dual basis and the lattice space thereby generated are called the reciprocal basis and reciprocal lattice; V is the volume of the elementary cell of the direct lattice, whereas its inverse is the volume of the elementary cell of the reciprocal lattice. These concepts in crystallography are of utmost importance: the constructive interference which provides a diffraction spectrum occurs only when the vector differences between the wave vectors of the incident and diffracted x-rays are vectors of the reciprocal space.
22.
This is in contrast to the formulation followed here and which, as archaic as it might seem, provides nonetheless a practical tool widely used by the working physicist.
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