two versions of 50 cm long plastic rulers

The first version of Rulers was made in 2000, the second in 2005. The first consisted of a set of 50 cm rulers shortened by 2 mm. The length of each millimetre graduation was reduced very slightly, creating a defect imperceptible to the human eye. In the second version, the 50 cm rulers were extended by 5.5 mm. This was a discrepancy that the human eye could notice. Subsequently I gave the rulers out to people, without pointing out the flaw.