Summary

My Qidi Tech X-Plus Printer was not printing square (no 90º angles on corners). The fix turned out to be easy (twist the whole X-axis carriage along the Z-axis until one or more gear teeth are skipped on the belt). What was difficult was verifying the fix.

I created this object to be printed two times, with one overlaid and on the other and rotated 90º to check squareness.