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rulers are usually accurate but cutting mats, not so much (even all four sides can vary) there have been cases where rulers were not accurate also, and even after the errors were noticed the incorrectly printed rulers showed up for sale at discount type stores.....
anything new that you buy check for accuracy!
Sad but true, not sure if bureau of weights and measures even exists anymore.
Will not even mention that some cloth measuring tapes can..... s t r e t c h.