Covariant Versus Contravariant
Subscript versus superscript:

Covariant potential vector.
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Contravariant potential vector.
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Greek indexes go from 0, 1, 2, 3.
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Roman indexes go from 1, 2, 3.
Memory aid: Co is a commie, commies are low, negative; the contras are proud, positive fighters, up against a wall.
Slide 30 believes in beliefs.