Four-vector

A four-vector V is defined as:

where the upper index denotes it to be contravariant. The covariant form can be obtained using the metric tensor g:

Often (but not always) the metric is diagonal, meaning the only difference between covariant and contravariant four-vector components are signs, though the signs depend on which metric is used.

Read more about Four-vector:  Four-position, Dynamics, Quantum Theory, Physics of Four-vectors