The notations in this article are: lowercase bold for three-dimensional vectors, hats for three-dimensional unit vectors, capital bold for four dimensional vectors (except for the four-gradient), and tensor index notation. A four-vector "A" is a vector with a "timelike" component and three "spacelike" components, and can be written in various equivalent notations:  where in the last form the magnitude component and basis vector have been combined to a single element. The upper indices indicate contravariant components. Here the standard convention is that Latin indices take values for spatial components, so that "i" = 1, 2, 3, and Greek indices take values for space "and time" components, so "α" = 0, 1, 2, 3, used with the summation convention. The split between the time component and the spatial components is a useful one to make when determining contractions of one four vector with other tensor quantities, such as for calculating Lorentz invariants in inner products (examples are given below), or raising and lowering indices. In special relativity, the spacelike basis E1, E2, E3 and components "A"1, "A"2, "A"3 are often Cartesian basis and components:  although, of course, any other basis and components may be used, such as spherical polar coordinates  or cylindrical polar coordinates,  or any other orthogonal coordinates, or even general curvilinear coordinates. Note the coordinate labels are always subscripted as labels and are not indices taking numerical values. In general relativity, local curvilinear coordinates in a local basis must be used. Geometrically, a four-vector can still be interpreted as an arrow, but in spacetime - not just space. In relativity, the arrows are drawn as part of Minkowski diagram (also called "spacetime diagram"). In this article, four-vectors will be referred to simply as vectors. It is also customary to represent the bases by column vectors:  so that:  The relation between the covariant and contravariant coordinates is through the Minkowski metric tensor (referred to as the metric), "η" which raises and lowers indices as follows:  and in various equivalent notations the covariant components are:  where the lowered index indicates it to be covariant. Often the metric is diagonal, as is the case for orthogonal coordinates (see line element), but not in general curvilinear coordinates.