There is another way of specialising coordinates which may be mentioned here for completeness. It is always possible to choose coordinates such that the determinant $g = -1$ everywhere (as in Galilean coordinates). This is explained in \SecRef{49}. We may also consider another class of specialised coordinates---those which are permissible in special problems. There are certain (non-Euclidean) coordinates found to be most convenient in dealing with the gravitational field of the sun, Einstein's or de~Sitter's curved world, and so on. It must be remembered, however, that these refer to idealised problems, and coordinate-systems \index{Coordinate-systems!natural}% \index{Coordinate-systems!proper}% with simple properties can only be approximately realised in nature. \PageSep{81} \index{Coordinate-systems!statical}% \index{Static coordinates}% If possible a \emph{static system} of coordinates is selected, the condition for this being that all the~$g_{\mu\nu}$ are independent of one of the coordinates~$x_{4}$ (which must be of timelike character\footnotemark).\footnotetext {$dx_{4}$ will be timelike if $g_{44}$ is always positive.} In that case the interval corresponding to any displacement~$dx_{\mu}$ is independent of the ``time''~$x_{4}$. Such a system can, of course, only be found if the relative configuration of the attracting masses is maintained unaltered. If in addition it is possible to make $g_{14}$, $g_{24}$, $g_{34} = 0$ the time will be reversible, and in particular the forward velocity of light along any track will be equal to the backward velocity; this renders the application of the name ``time'' to~$x_{4}$ more just, since one of the alternative conventions of \SecRef{11} is satisfied. We shall if possible employ systems which are static and reversible in dealing with large regions of the world; problems in which this simplification is not permissible must generally be left aside as insoluble--e.g.\ the problem of two attracting bodies. For small regions of the world the greatest simplification is obtained by using natural coordinates. \Section{37.}{Einstein's law of gravitation} \index{Einstein's law of gravitation}% \index{G@$G_{\mu\nu}$ (Einstein tensor)}%