At NMISA the SI unit for length is realised by an iodine stabilised He-Ne laser, anoptical measurement. In industry most measurements are performed by mechanical probing.Gauge blocks are the link between the optical measurements and the mechanicalmeasurements, and are therefore critical in disseminating traceability. The gauge blocklength is currently determined by wringing the gauge block onto a platen. This islaborious, requires skill and causes contact errors. Since around 1943, there have beenefforts to build a double ended interferometer where gauge block length can be determinedwithout wringing it onto a platen. While there has been a lot of progress in buildingDEIs, to date, there is still no commercially available DEI. We present a collaborationproject between the National Metrology Institute of South Africa (NMISA), the NationalInstitute of Standards (NIS Egypt) and Stellenbosch University (SUN) in South Africa tobuild a double ended interferometer for use at the respective National MetrologyInstitutes. We investigate improvements to the interferometric calibration of gauge blocksand recent developments. The different systems currently in use in the national metrologylaboratories are described and various designs are investigated. Lastly the expectedoutcome of the project will be discussed.