The surface plate. Modern precision all stems from the concept of a flat reference. However, the technique to make a surface as flat as possible (which is based on simple geometry) was first discussed in the early 19th c. In terms of profound impact on modernity, the metal screw-cutting lathe.
The metal lathe is not really a 'modern' invention is it?
https://en.wikipedia.org/wiki/Screw-cutting_lathe#Modern_scr...