Now, the first thing we need to do to have any semblance of repeatability is to establish a reference plane.
Straight edges and level surfaces, similar to points, can be gotten from the standards of math and don’t need any reference standard.
For instance, a level surface can be created by lapping three surfaces together. Lapping means essentially rubbing two surfaces together. The initial two surfaces are scoured together to make smooth surfaces, it is anyway feasible for one of these to be inward and the other curved as a result of imperfect motion while lapping by hand. When we have lapped two surfaces together to sufficient smoothness a third surface is then lapped against the second. When the third mix of two surfaces is lapped together this will eliminate any shape and produce a level surface. This cycle is iterated until the necessary evenness is accomplished. This operation can produce an extremely flat surface, the surface where a speck of dust is like a huge mountain. Now we can start taking accurate measurements from our reference plane.