In Finland, approximate measures derived from body parts and were used for a long time, some being later standardized for the purpose of commerce. Some Swedish, and later some Russian units have also been used.
- vaaksa – The distance between the tips of little finger and thumb, when the fingers are fully extended.
- kyynärä – c. 60 cm – The distance from the elbow to the fingertips.
- syli – fathom, c. 180 cm – The distance between the fingertips of both hands when the arms are raised horizontally on the sides.
- virsta – 2672 m (Swedish), 1068.84 m (Russian)
- peninkulma – 10.67 km – The distance a barking dog can be heard in still air.
- poronkusema – c. 7.5 km – The distance a reindeer walks between two spots it urinates on. This unit originates from Lapland.
- leiviskä – 8.5004 kg
- kappa – 5.4961 l
- tynnyrinala – 4936.5 m2 – The area (of field) that could be sown with one barrel of grain.
- kannu – 2.6172 l
- kortteli – 148 mm (length) or 0.327 l (volume)
The French Revolution gave rise to the metric system, and this has spread around the world, replacing most customary units of measure. In most systems, length (distance), mass, and time are base quantities.
Later science developments showed that either electric charge or electric current could be added to extend the set of base quantities by which many other metrological units could be easily defined. (However, electrical units are not necessary for such a set. Gaussian units, for example, have only length, mass, and time as base quantities, and the ampere is defined in terms of other units.) Other quantities, such as power and speed, are derived from the base set: for example, speed is distance per unit time. Historically a wide range of units was used for the same type of quantity: in different contexts, length was measured in inches, feet, yards, fathoms, rods, chains, furlongs, miles, nautical miles, stadia, leagues, with conversion factors which were not powers of ten. Such arrangements were satisfactory in their own contexts.
The preference for a more universal and consistent system (based on more rational base units) only gradually spread with the growth of science. Changing a measurement system has substantial financial and cultural costs which must be offset against the advantages to be obtained from using a more rational system. However pressure built up, including from scientists and engineers for conversion to a more rational, and also internationally consistent, basis of measurement.
In antiquity, systems of measurement were defined locally: the different units might be defined independently according to the length of a king’s thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in hands and knuckles. The unifying characteristic is that there was some definition based on some standard. Eventually cubits and strides gave way to “customary units” to meet the needs of merchants and scientists.
In the metric system and other recent systems, a single basic unit is used for each base quantity. Often secondary units (multiples and submultiples) are derived from the basic units by multiplying by powers of ten, i.e. by simply moving the decimal point. Thus the basic metric unit of length is the metre; a distance of 1 m is 1,000 millimetres, or 0.001 kilometres.