There do exist

- other
*position units*than the metre, for example*miles*,*inches*,*yards*and*feet*, - other
*time**units*than the second, for example*minutes*,*days*and*centuries*and thus also - other
*speed**units*than the metre-per-second, for example*miles-per-hour*.

It might be smart to pick one unit for each property as a “standard” unit. People may use other units if they want, but at least we will all have some common ground. Let’s choose some default units for the fundamental properties and call them **fundamental units**:

Fundamental units^{[1]} |
||
---|---|---|

Position | Metre | $\mathrm m$ |

Time | Second | $\mathrm s$ |

Quantity | $-$ | $-$ |

Mass | Gramme | $\mathrm g$ |

Temperature | Kelvin | $\mathrm K$ |

Charge | Coulomb | $\mathrm C$ |

Luminosity | Candela | $\mathrm{cd}$ |

We did not give ‘quantity’ a unit as it is just a unit-less count (see the Existence discipline).

Prefixes don’t matter; *kilo*metres and *nano*seconds just mean ‘1000 metres’ and ‘a billionth second’. They are still fundamental units, just multiplied a couple of times with prefixes. So,

- $\mathrm m$ and $\mathrm s$ are
**fundamental units**for position and time, - $\mathrm{km}$ and $\mathrm{\mu s}$ (microsecond) are as well, just with prefixes, whereas
- $\mathrm{mile}$ and $\mathrm{hr}$ (hour) are
**not**fundamental units (they are not just the metre or second with a prefix).

Combined (derived) properties naturally have *combined *(*derived*) fundamental units, such as metres-per-second $\mathrm{m/s}$ and cubic metres $\mathrm{m^3}$.

- $\mathrm{m/s}$ and $\mathrm{m^3}$ are
**fundamental units**for velocity and volume, - $\mathrm{km/s}$ and $\mathrm{mm^3}$ (cubic millimetre) are as well, just with prefixes, whereas
- $\mathrm{km/hr}$ and $\mathrm{L}$ (litre) are
**not**fundamental units.

Note that all this was just a choice from our side. We could just as well have chosen the ‘hour’ as the fundamental time unit instead of the ‘second’. It wouldn’t matter, we just had to choose something. Our choice of units here matches the units in the official, international so-called **SI**^{1} **unit system**.^{2} See an overview and more description of the SI system in Resource: The SI unit system.

In everyday life other units are often used (highly depending on country and culture). We buy milk in *litres* or *gallons*, use *minutes* and *hours*, dress according to *degree Celsius* or *Fahrenheit* etc. In professional and educational contexts, we see units invented for convenience such as *nautical miles* on the seas, *pinches *and *teaspoons *in the bakery and *lightyears* in space.

An overview of several often-used non-SI units can be found in Resource: Often-used units.

References:

- ‘
**The International System of Units (SI)**’ (book),*B. Inglis, J. Ullrich and others*, Bureau International des Poids et Mesures (BIPM), 9th ed., 2019, www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf, chapter 3 - ‘
**Guide for the Use of the International System of Units (SI)**’ (article),*Ambler Thompson & Barry N. Taylor*, National Institute of Standards and Technology, 2008, physics.nist.gov/cuu/pdf/sp811.pdf