The base unit of length is the metre.
The SI system is based on seven base units, all the other units are derived from these. Length is on of the base units, and is measured in metres. The prefix system is a shorthand way of writing fractions and multiples of the base units. A millimetre is 0.001 metres, the milli- just denotes the number of decimal places, the distance is still being measured in metres but is written as millimetres to save on wrting or pronouncing all the zeros. millimetres and kilometres are still metres, the prefix merely indicates the thousandths or thousands of metres.
All distances are measured in metres, sometimes fractions or multiples of metres are used for ease, but the measurement is still in metres.
The answer depends on the scale of the measurement. Distances between cities is measured in kilometres. But a kilometre is no good for measuring the distance between consecutive peaks in a light wave. Here a nanometre is more appropriate.
Let's see..... There's millimeters, centimeters, inches, feet, yards, meters, kilometers, miles, and others that I can't quite recall at the moment.
The meter
The Metre.
A square meter (m2) is a derived unit of measure of surface area in the metric system. A yard is a traditional English unit of measure of distance.
Metre
The metric system
It depends on the distance between the two points. If it is more than a kilometer apart then you would use kilometer. Unless you wanna do it with centimeter. It's just whatever is easier.
In the earlier times, there was no unit to measure the distance and thus there was no symmetry. To fight this problem, in 1790, french developed a standard unit of measurement called the metric system.
There is no individual unit in the Metric system that is near one inch in the English system, I'm afraid.
The SI unit for mass in the metric system is the kilogram.
Meters
The Metre.
Distance is measured metrically in meters
A square meter (m2) is a derived unit of measure of surface area in the metric system. A yard is a traditional English unit of measure of distance.
A global positioning system (GPS)
Metre
The metric system
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It depends on the distance between the two points. If it is more than a kilometer apart then you would use kilometer. Unless you wanna do it with centimeter. It's just whatever is easier.