Large calibration matrix (145mm x 145mm) to identify backlash and other hard to resolve print quality issues.

Print and play. Will give you a good idea of how your print quality varies over the x and y limits of the bed, and also of the first 20mm of the Z axis as well.

This was very helpful for me in identifying defects in printed pulleys or shafts - eg errors that are introduced at very specific intervals or only in specific ares of the print bed.

Obviously, you must start with a level print surface, the vertical and horizontal lines are 1mm.