00 is the highest and 3 is the lowest.

Take the knife-edge square as an example:

Class 00 is closest to the right angle, which may be a few seconds apart, but it is not as accurate as the angle gauge of class 1 and cannot be used as the standard for measuring tools. I have never seen a 00-level edge square, and it is rarely sold in the market.

The level of 0 is almost the same, which is the most commonly used fitter exam and the highest level on the market. There may be a gap of more than ten seconds, and the fitter competition is enough, but only the big factory can ensure the quality.

1 grade is worse than the junior fitter, and the intermediate fitter exam can barely be used. It is best not to use it above the senior fitter. There will be a bright diagonal when measuring the proportion by angle.

There seems to be no such low-grade knife edge square in Grade 2 and Grade 3, which is not much different from ordinary steel square.

Knife edge ruler is used to measure verticality, and gestures are very important. If the square is a little crooked, it will be much worse. Vision is more important. You don't know how bad the workpiece is until you look at it, or you don't feel anything at all. No matter how accurate the square is, it's useless.