  In axistationary asymptotically flat spacetimes zero angular momentum
observers (ZAMOs) define an absolute standard of non--rotation locally, as can
be verified by the absence of any Sagnac effect for these observers.
Nevertheless, we argue that on a global scale the only physically meaningful
concept is that of relative rotation. The argument is substantiated by solving
Einstein's equations for an approximate thin shell model where we keep a degree
of freedom by relaxing the natural assumption of vanishing rotation at
asymptotic infinity at the outset of the analysis. The solution reveals that
Einstein's equations only determine differences in the rotation rate of ZAMOs,
thereby establishing the concept of relative rotation globally. The
interpretation of rotation as relative in a global context is inherently linked
to the freedom to transform between coordinate systems rotating relative to
each other, implying that an arbitrary ZAMO located at any radius may claim to
be the one who is non--rotating on a global scale and that the notion of an
asymptotic Lorentz frame relative to which one may measure absolute rotation is
devoid of any meaning. The concept of rotation in Kerr spacetime is then
briefly discussed in the context of this interpretation.
