Rulers are used to measure the length of an object in either centimeters or inches. Metric rules use centimeters. English rulers use inches.
Regardless of the unit that is being used, the first step in obtaining an accurate measurement of an object’s length is making sure that one end of the object aligns perfectly with the first “slash” on the ruler.
Sometimes metric rulers count by every ten centimeters. In this situation, there are often ten slashes between every numerical value written on the ruler. Therefore, every slash has a value of one centimeter. Thus, one centimeter would be added for every slash over a numerical value on the ruler.
Other times, metric rulers show ten slashes between every one centimeter. In this case, every slash would have a value of .1 centimeter (or one millimeter). Therefore, .1 centimeter would be added for every slash above a number written on the ruler. English rulers use inches.
On most English rulers, there are eight slashes between every inch. Therefore, each slash has a value of 1/8 of an inch. Thus, 1/8 inch is added for every slash there is over a number on an English ruler.