Length can be measured in units such as metres and kilometres. They are part of the metric system. In the imperial system, units such as feet and miles are used. Time can be measured in units such as minutes and hours, and they are neither part of the metric nor the imperial system.
In the metric system, units such as centimetres and millimetres are used for small lengths. However, smaller units can also be expressed. A micrometre is a very small unit, one millionth of a metre, and a nanometre is one billionth of a metre.
Time can also be measured in very small units. A millisecond is one thousandth of a second. The time of 9.375 seconds consists of nine full seconds and 375 milliseconds. A microsecond is one millionth of a second and a nanosecond is one billionth.
Very small units of length and time can be expressed with the prefixes -micro and -nano. The prefix -micro is used in microseconds and micrometres and means one millionth. For even smaller units, the prefix -nano can be used as in nanoseconds and nanometres. This means one billionth.
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