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Meters, feet, inches... all are standard units of measurement, meaning they can be referred to and serve as a means of comparison for two measurements made separately. However, in order to use these standard units, one needs calibrated measuring equipment... like a ruler or a metric tape.
In the absence of a calibrated equipment, one can make measurements with non standard units, such as measuring a distance by counting "steps" or determining the length of an item by aligning other objects, such as paper clips. For example, one can say a pencil is "5 paper clips" long. This kind of measurement has the advantages of being easy to perform and a fast way to introduce measurement science to students and pupils.
By using concrete objects as the unit of measure (paper clips) instead of an abstract unit of measure (inches or centimeters) it has didactic advantages as well. The downside is, the length of the paper clip itself being non-standard (there are several types of paper clips available), it has limited practical usage, beyond simple comparison. Actually, explaining the limitations of non standard measuring units is a good way to introduce standard measuring units - be they Imperial or Metric.
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