  A logic of reciprocity between inertial frames in relative uniform motion is
investigated. Relativity allows any reference frame to apply Lorentz
Transformation while reciprocity would require the relative frame to use
Inverse Transformation for the same event, and vice versa. After such
transformations, an inseparability between relativistic measurements and the
covariant scale is examined. Scale conversion has been found to be necessary to
preserve spacetime invariance in Special Relativity. A new derivation of the
Lorentz Factor has been found based on the scale effect. The physical meaning
of the Factor is that it serves as a scale conversion ratio between relative
reference frames. Reciprocity in relativity and causality of covariance in
relative motion are thereby conserved. These logical conclusions harmonize
intrinsic natural invariances vis-a-vis apparent observational covariances.
