A metric ruler is based on the International System of Units (SI), sometimes called the metric system, and is divded into either millimeters or centimeters instead of inches. Rulers are often 30 centimeters long, which are designated by large numbers on the ruler. Between each centimeter (cm) mark, there should be 10 smaller marks called millimeters (mm).  Make sure you read the ruler from left to right. If you are measuring an object, align it with the left side of the zero mark on the ruler. The left side of the line where the object ends will be its measurement in centimeters. This way the line thickness will not affect the measurement. Unlike with the English ruler, the measurements for the metric ruler are written in decimals instead of fractions. For example, 1/2 a centimeter is written as 0.5 cm. The large numbers next to the longest lines on the ruler denote the centimeter marks. A metric ruler has 30 of these marks. For example, place the bottom of a crayon on the far left side of the ruler to measure it. Note where the tip falls. If the crayon ends directly on the long line next to the large number 14, your crayon is exactly 14cm long. halfway between each centimeter, there is a slightly shorter line that denotes 1/2 of a centimeter, or 0.5cm. There are a total of 60 of these marks on a 30 cm ruler.  For example, you measure a button and the edge ends on the fifth line right between the 1 and 2 centimeter marks. Your button is 1.5cm long. For example, to measure 0.6 cm, count one thick line (5 mm) and one thin line (1 mm). Between each 0.5cm line, there are four additional lines that denote the millimeter marks. There are a total of 10 lines per centimeter, with the 0.5cm line acting as the 5 millimeter mark, making each centimeter 10mm long. There are 300 millimeter marks on a 30 cm ruler. For example, if you measure a piece of paper and it ends on the 7th mark between the 24 and 25 centimeter mark, it means your object is 247mm, or 24.7cm, long.

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Write a summary of the previous text in Vietnamese:
Lấy thước đo mét. Tìm hiểu vạch centimet. Tìm hiểu vạch 1/2 centimet. Tìm hiểu vạch milimet.