The PTB as the national metrology institute of Germany offers traceable pattern placement calibrations on 2D objects, primarily photomasks. The calibrations are performed by means of an optical mask comparator, the LMS 2020 from Leica company, with measurement uncertainties of 35 nm for the positions of patterns on, for example, high quality 152 mm masks. By far the largest uncertainty contribution for pattern placement measurement is due to the traceability to the SI unit of length. For interferometry in air, traceable length measurement puts high demands on the determination of the refractive index of air. This was, amongst other reasons, one point for the PTB to start the development of a new measurement system, the Electron Optical Metrology System (EOMS), which utilizes laser interferometry in a vacuum. The basic components of the EOMS will be described and a discussion about the different contributions entering the uncertainty budget is given on the basis of recent investigations. Latest pattern placement measurements on 1 micrometers structures on a 6 inch photomask showed reproducibilities from 5 to 7 nm.