An "inertial frame" has an inertial reference vector to a fixed point in the spacetime continuum. For example, suppose I place an object on a horizontal line and extend the line upwards. The line originates at an point x at the center of vertical symmetry in the plane perpendicular to the horizontal plane (and the line continues downwards to the bottom of the vertical line) at x = −X where x is the horizontal line velocity on my line.