The little lines on a metric ruler mark the millimeters between centimeters, while the little lines on an English ruler mark the eighths and 16ths between inches. Some English rulers may only have lines at every 1/2 inch or 1/4 inch.
To measure using a metric ruler, a person counts the number of marks after the last full centimeter measured. For example, if six marks are counted after centimeter 2, then the measurement taken is 2.6 centimeters.
An English ruler requires knowledge of the different fractions. Every inch is marked by a larger line and a number. The 1/2 inch is the center line between two set inches, and the 1/4 inch is the center line between two set 1/2 inches. The smallest lines are typically an 1/8 inch or 1/16 inch. Example measurements on an English ruler include 4 1/2, 6 3/8 and 9 7/16.