Unit millimeter. Post-it notes are a kind of post-it notes, which are sticky on the back and usually very small. It was founded by Arthur Foley in 1974. Usually, the default specification is in millimeters, and 10x 10 is a square with a side length of 10mm. Of course, there are also units in centimeters. If the unit is cm, then it is a square with a side length of 10 cm.