Since many authors call simultaneity between events a convention, and that under the specific set of rules we may choose a convention or a coordinate system relative to an IRF (or non-inertial) to describe space time, I wonder what's the relation between this and the effects of length contraction and time dilation. Are lengths contracting and time running slower relative to an IRF also conventions, and by that I don't mean that the total elapsed time between events is a convention also. Suppose that in the case of the twin paradox we use a different convention, like Dolby and Gull's radar coordinates. Since the lines of simultaneity are different, this must imply that time dilates differently and lengths contract differently than in the original setup. So are length contraction and time dilation also conventions, and we may choose how clocks run and rods contract, depending on the reference system we use, or the coordinate chart we use. Regards.