A ruler that is exactly right on paper. Unlike the screen version this needs no calibration, because a millimetre on a printed page really is a millimetre.
Before you print, set scaling to 100%. Most print dialogues default to “fit to page”, which shrinks the sheet by a few per cent and makes every ruler on it wrong. Look for Scale, Size or Page Sizing and choose 100%, Actual size, or None.
Measure this square. If it is not 50mm across, your printer scaled the page and every ruler on it is wrong by the same amount. Print again with scaling set to 100%, not “fit to page”.
30 cm, in millimetres
Cut out 2 strips and join them end to end. No page is 30cm across, so a single strip that claimed to be one would not be.