2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202160Ideal rods and clocks are defined as an infinitesimal symmetry of the spacetime. Since no a priori geometric structure is considered, all the possible models of spacetime are obtained.Part of the results of this article has been announced at the meeting ``XXIV International Workshop on the Fundamental Problems of High Energy Physics and Field Theory'' 27-29 June 2001, Protvino, RussiaGeneral Relativity and Quantum Cosmologytext