**Traceability** can be a property of a measurement result, which is said to be **traceable** if the measured quantity can be **traced** back to a primary measurement standard through a chain of calibrations. Such primary standards for various quantities are usually provided by national **metrology** institutes (NMI).

**Length measurements** are traced back to the unit **metre**, which in turn is defined by the speed of light in vacuum *c* and the unit **second**: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second".

**A Nanometre (nm)** is the 1 / billion (10^{-9}) part of a metre. **Sub-nm** means even smaller than 1 nm. The need for improved **accuracy** in this range is already there or on the horizon for certain applications in metrology and industry.

**This EMRP project** aims to improve the traceability of **laser interferometers** and **capacitive sensors**, which are the most widely used measurement devices to measure lengths or displacements with utmost precision and accuracy today. The starting point is to develop a better understanding of the underlying effects which limit the accuracy and to develop and to validate **theoretical models** in order to calculate **corrections** to reduce **measurement uncertainty**.