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Find an object or distance between two points you want to measure.  This can be a length of wood, string, or cloth, or a line on a sheet of paper. Rulers and yardsticks are better to use on hard flat surfaces. If you are measuring a person for clothing it is best to use a flexible tool like a seamstress' tape. Long distances can be measured using a measuring tape. This will usually be on the left side.   Make sure the end of the ruler is flush with your object. Use your left hand to hold it in place. Use your right hand to adjust the other end of the ruler. You will now read the ruler to see how long the object is.   Read the last number on your ruler that is alongside the object.  This will indicate the "whole unit" length of the object, example: 8 inches. Count the number of fraction marks (dashes) the object you are measuring goes beyond the last whole number. If your ruler is marked in 1/8 inch increments, and you are 5 marks past the last whole unit number, you will be 5/8 inches beyond the 8, and your length will be read "8 and 5/8 inches'. Simplify fractions if you are able. For example, 4/16 of an inch is the same as 1/4 inch. You will be reading measurements in units of 10, following the metric system.   Read the larger hashmarks as centimeters. Go to the nearest centimeter line. That will indicate the "whole unit" length. For example, 10 centimeters. In the case of a metric rule marked in centimeters (cm), read the intermediate marks as millimeters (mm). Read how many intermediate marks go past your whole unit measurement to the edge of the object. For example, if you measured an object that was 10cm plus 8mm, your measurement would be 10.8cm. A retractable steel measuring tape would work best for this.  Slide the zero end of the tape against one wall, or have a helper hold it, then pull out enough tape to reach the opposite wall. Here, you should have two sizes of numbers, the larger for feet (or meters), the smaller, for inches (or centimeters). Read the feet (or meters) first, inches (or cm), then fractions thereof. Example, a distance may read "12 feet, 5 and 1/2 inches". You can also use rulers as straight edges in art or geometry.  Lay it down on the surface you are drawing on, and lay your pencil point along the edge of the rule. Use your ruler as a guide for a straight edge. Keep holding the ruler steady to get the straightest line possible.
Measure with a ruler or tape measure. Place the zero end of your rule at the end of your object. Move to the opposite side of the object you are measuring. Use a metric or decimal rule with a metric ruler. Use a tape measure to measure between objects, for instance, walls. Use your 12 inch rule (or similar instrument, like a yardstick) to draw a straight line.