* Metric: Rulers are typically marked in centimeters (cm) and millimeters (mm). There are 10 millimeters in 1 centimeter. So 0.66 would be 6.6 millimeters (0.66 x 10 = 6.6).

* Imperial: A standard ruler is divided into inches, with each inch divided into 16ths, 8ths, 4ths, and halves. 0.66 is equivalent to 2/3 of an inch. You would find this mark between the 2/4 (half inch) mark and the 3/4 mark on a standard ruler.

Let me know if you have any other ruler questions!