A millimeter (mm) is the smallest. In order from smallest to largest they rank millimeter, centimeter (cm), decimeter (dm), and meter (m). These are all common units of length in the metric system.
A millimeter is one-thousandth of a meter. A centimeter is one-hundredth of a meter. A decimeter is one-tenth of a meter. The metric system was created about 200 years ago by French scientists who wanted to simplify measurements. Every measurement unit is a factor of ten smaller or larger than the closest other measurement unit. This makes it easy to convert between different metric units of measurement. The prefix of any measurement describes the relationship between the main measurement. For example a milliliter is also one-thousandth of a liter.