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Non-standard units
Humans have always been able to measure things even before the systemisation of measurement by comparing one item to another. This is still widely practiced today, although the comparisons obviously change. Some of these non-standard measurement units, sometimes found in books etc., include:
 * A ton of TNT, and its multiples the kiloton, the megaton and the gigaton. Often used in stating the power of very energetic events such as explosions and volcanic events and earthquakes and asteroid impacts. A gram of TNT as a unit of energy has been defined as 1000 thermochemical calories = roughly 4184 joules.
 * The Hiroshima atom bomb. Its force is often used in the public media and popular books as a unit of energy. (Its yield was roughly 13 kilotons.)
 * The weight of an elephant. It is often used as a unit of weight in popular books about very big animals such as dinosaurs. This unit needs to be defined, as the real weight of elephants varies much with age and sex and species.

Systems of measurement in the ancient world
Prior to the global adoption of the metric system many different systems of measurement had been in use. Many of these were related to some extent or other. Often they were based on the dimensions of the human body and were effectively a standardisation of previously used measures.

In the History of measurement many of the units that we have records of, or at least the ones that have been used in Europe and around the Mediterranean are variations on older systems originating in the Ancient Near East and Egypt.

Ancient systems of measurement

 * Mesopotamian
 * Egyptian Based on Mesopotamian, developed around 3000BC. Cubit was intl standard
 * Persian based on mesopotamian, first persian empire c650BC
 * Greek Derived from egyptian and Mesopotamian and Persian
 * Roman Based on Greek, also egyptian influences
 * Hebrew very similar to babylonian system. whatever that is.

Mediæval measurements
Medieval measures can be categorized by ever expanding commercial, political and religious spheres of influence.

Eastern European
In Eastern Europe traditional standards of measure were predominantly of Greek origin
 * Polish system
 * Romanian system
 * Russian system
 * Tatar system

Western and Northern European
In Western and Northern Europe traditional standards of measure were predominantly of Roman origin
 * Spanish system
 * French system
 * English system
 * Scottish system
 * Finnish system
 * Danish system
 * Norwegian system
 * Swedish system
 * German system
 * Dutch system

Unrelated Historical systems of measurement

 * The Arabic system developed from the Persian system.
 * Indus Valley developed c2600BC
 * Vedic Used in Vedic civilisation (began around 1000BC), still used today in Hinduism and Jainism
 * Chinese Used since 13thC BC, still used today, though adapted to fit metric system.
 * Japanese Developed from the Chinese System, officially adopted 701BC

Imperial and US customary units
Both the Imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth (Commonwealth of Nations) and the former British Empire. They are still used to some extent but have now been mostly replaced by the metric system. US customary units are the main system of measurement in the United States however some steps towards metrication have been made. The metric system is prefered in certain fields such as science, medicine and technology.

These two systems are closely related. Units of length and area (the inch, foot, yard, mile, acre, etc.) are identical except for surveying purposes. Units of mass and weight differ for units larger than a pound (lb). The Imperial system uses a stone of 14 lb., a long hundredweight of 112 lb. and a long ton of 2240 lb. The stone is not used in the US and the hundredweights and tons are short being 100 lb. and 2000 lb. respectively.

Where these systems most notably differ is in their respective units of volume. A US fluid ounce (fl. oz.) is slightly larger than its Imperial equivalent (the former being approximately 29.6 ml and the latter 28.4 ml). However, as there are 16 US fl. oz. to a US pint as opposed to the 20 Imperial fl. oz. per Imperial pint these pints are quite different in volume. The same is true of quarts, gallons, etc. Six US gallons are a little less than five Imperial gallons.

The metric system
Metric systems of units have evolved since the adoption of the first well-defined system in France in 1791. During this evolution the use of these systems spread throughout the world, first to the non-English-speaking countries, and more recently to the English speaking countries.

Multiples and submultiples of metric units are related by powers of ten; the names for these are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.

In the early metric system there were two fundamental or base units, the metre and the kilogram, for length and mass. The other units of length and mass, and all units of area, volume, and compound units such as density were derived from these two fundamental units.

Mesures usuelles (French for customary measurements) were a system of measurement introduced to act as compromise between the metric system and traditional measurements. It was used in France from 1812 to 1939. Similar proposals to metrify the English system of measurement have been put forth but never officially adopted.

A number of variations on the metric system have been in use. These include gravitational systems, the centimetre-gram-second systems (cgs) useful in science, the metre-tonne-second system (mts) once used in the USSR and the metre-kilogram-second system of units (mks) most commonly used today.

The current international standard metric system is the International System of Units (Système international d'unités or SI) It is an mks system based on the metre, kilogram and second as well as the Kelvin, Ampere, candela, and mole.

The SI includes two classes of units which are defined and agreed internationally. The first of these classes are the seven SI base units for length, mass, time, temperature, electric current, luminous intensity and amount of substance. The second of these are the SI derived units. These derived units are defined in terms of the seven base units. All other quantities (e.g. work, force, power) are expressed in terms of SI derived units.

Natural units
The above systems of units are based on arbitrary unit values, formalised as standards. Some unit values occur naturally in Science. Systems of units based on these are called Natural units. The following are examples of these system of natural units.


 * Planck units
 * Geometrized units

Atomic units
Atomic units (au) are a convenient system of units of measurement used in atomic physics, particularly for describing the properties of electrons. The atomic units have been chosen such that the fundamental electron properties are all equal to one atomic unit.

Currency
For main article see currency

A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country or a fraction thereof. The most frequently used currencies in the world today are the US dollar (and US cent (1/100 of a dollar)) and the Euro (and Eurocent).