This paper presents the definition of a coordinate frame, entitled the principal frame $(PF)$, that is useful for metric calculations on spatial and planar rigid-body displacements. Given a set of displacements and using a point mass model for the moving rigid-body, the $PF$ is determined from the associated centroid and principal axes. It is shown that the $PF$ is invariant with respect to the choice of fixed coordinate frame as well as the system of units used. Hence, the $PF$ is useful for left invariant metric computations. Three examples are presented to demonstrate the utility of the $PF$.

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