Meters (m) are the largest unit used on a ruler and are marked every 100 cm on the ruler.
Centimeters (cm) are the major markings on most everyday metric rulers. They are marked every 10 mm and are normally accompanied by a number indicating the distance from the start of the ruler scale.
Millimeters are the smallest unit generally used on a metric ruler. Millimeters are marked without numbers on everyday rulers. Every fifth millimeter is marked with a longer line marker, indicating the half centimeter mark.
Decimeters are not used as commonly on everyday rulers and are the equivalent of 100 mm or 10 cm.
Metric rulers are usually constructed from plastic, wood or metal with the markings on the top side, which is sometimes transparent on the plastic versions. The most common ruler sizes are 15cm, 30cm and 100cm, although rulers can also be found on computer screens as layout tools and as roll-out tapes for measuring longer distances.