I find the previous answer confusing. Inches are never subdivided into centimetres or millimetres. What the divisions in each inch represent depends on the scale being used. Most often they are tenths of an inch, but can be eighths, 12ths or 16ths. The simplest way to find out which is being used is to count the sections between the inch markings on the ruler you are using.

The Marks Between 1 Inch And 2 Inch etc on the standard ruler represent the centimeter and milimeter respectively.The milimeter are the smallest marks.1 inch is equal to 2.54 centimeters.