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Realizing traceability to the SI in dimensional metrology: today and in the future

The paper deals with a problem of increasing importance. That is in which way the technologies and formalism can be changed to establish traceability in a reasonable and worldwide acceptable direction in future. Until 1960, when the Pt-Ir-protype in the BIPM was the definition and realization of the SI-unit 'metre', formal and physical reasons called for hierarchical calibration chains from the BIPM via the National Metrology Institutes into the national societies and industries. The procedure is well proven and formally well accepted. Since the definition of wavelengths of discharge lamps- and moreover of laser wavelengths of frequency stabilized lasers - there is no physical reason further on to stay with the traditional way of realizing traceability for length and dimensional metrology. Physical principles and well proven of technologies (mise en pratique) allow to everybody , everywhere at any time to realize the SI unit for length. Experiences of NMI's show that the vacuum wavelength of lasers has never been an accuracy limiting factor for shop floor measurements. Other technical and formal aspects are of more relevance and need to be dealt with successfully to realize direct traceability as an acceptable alternative to the hierarchical calibration chain.