Abstract: The datum line establishing tool provides an improved method for deriving an accurate datum line on Printed Circuit Boards (PCB) and other similar assemblies without the need for having close tolerance PCB index holes and base tool pins. The datum line establishing tool includes a tool base and a spherical pin assembly (consisting of a sphere attached to a pin base); and a cylindrical section pin assembly (consisting of a cylindrical section to a second pin base); and a compression spring assembly. The two pin assemblies are inserted into their respective tool base cavities and are held in place with an upward bias by the compression spring assembly against the cavity flange. An improved method is disclosed by deriving an accurate datum-line on printed circuit boards (PCB) without the need for having close tolerance index holes and corresponding index pins. The process comprises the steps of placing the index holes of a PCB over two pin assemblies.