There exist several unit prefixes used like the SI prefixes, but that are not part of the SI system. An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple or submultiple. ... Cover of brochure The International System of Units. ...

Some of these were never part of any actual system of measurement. A few were proposals that were rejected. None are in common use, and many of the powers of ten they represent are already taken by SI prefixes.

One such prefix is bronto, used in the fake term brontobyte. References on the World Wide Web suggest meanings of the bronto prefix to be variously any of 10¹⁵, 10²¹, 10²⁴, or 10²⁷. SI has already produced standard prefixes for 10¹⁵ (peta), 10²¹ (zetta) and 10²⁴ (yotta). This article describes the SI prefix peta. ... Zetta (symbol Z) is a SI prefix in the SI (system of units) denoting 1021 or 1 000 000 000 000 000 000 000. ... Yotta (symbol Y) is a SI prefix in the SI (system of units) denoting 1024 or 1 000 000 000 000 000 000 000 000. ...

The prefix guaca is a Chemistry joke arising from these facts:

• Guacamole is made of avocado, which sounds (with apologies to the learned professor) remarkably like Avogadro.
• Guacamole is spelled (if not pronounced) remarkably like yoctomole.
• A mole is made of Avogadro's number of molecules.
• A single molecule is made of about 1.66 yoctomoles.
• A yoctomole is about six tenths (0.602) of a molecule.
• A guacamole must be one molecule, much more convenient.
• From this it follows that guaca must be .

There are many other unofficial prefixes. Some of them appear to come from a numerical Greek root indicating the power of 1000 the prefix means. These prefixes are as follows: Guacamole Guacamole is an avocado-based relish or dip originating from the time of the Aztecs. ... Binomial name Persea americana Mill. ... Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto, (Turin August 9, 1776 - July 9, 1856) was a great Italian scientist. ... Yocto (symbol y) is a SI prefix in the SI system of units denoting a factor of 10-24. ... Avogadros number, also called Avogadros constant (NA) is a large constant used in chemistry and physics. ...

A proposal of Jim Blowers can be found here. This describes the naming scheme as follows:

zetta is z + -etta, which is an alteration of septi-, meaning 7, as 21 is 7 groups of three. yotta is y + -otta, an alteration of octo-, meaning 8. The pattern here is that we go backwards from the beginning of the alphabet, starting with z and y, and we follow it up with an alteration of the Greek or Latin for the next number. According to this pattern, the next ending should be xona-, since x comes before y in the alphabet, and 9 is noni- in Latin. Similarly, 10³⁰ should be weka-, since w precedes x and 10 is deka in Greek.

There have also been proposals to use the units of one of these systems with prefixes of a different base than ten, e.g. binary, dozenal or hexadecimal. In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten. ... Duodecimal is a base 12 numeral system. ...

Obsolete prefixes

There are some prefixes that were used in some older versions of the metric system but that are not part of SI. The prefixes 'myria' and 'myrio' came from the Greek μύριοι (mýrioi), meaning both ten thousand and "an indefinite large number". There is no Latin equivalent. Adopted by the French in 1795, they were not retained when the SI prefixes were agreed internationally by the 11th CGPM in 1960. The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ... Cover of brochure The International System of Units. ... These numerical prefixes are from the Latin language: Whole numbers 1. ... 1795 was a common year starting on Thursday (see link for calendar). ... An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple or submultiple. ... The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, sometimes written in English Conférence Générale des Poids et Mesures). ... 1960 (MCMLX) was a leap year starting on Friday (the link is to a full 1960 calendar). ...

They are still sometimes used as a prefix for constructing words:

• Myriad = 10,000 of anything (often used figuratively to mean "a great many" in English)
• Myriagon = a 10,000 sided polygon.
• Myriapoda = a group of creatures including the centipedes and millipedes.

Also obsolete are metric double prefixes, such as those formerly used in micromicrofarads, hectokilometres, micromillimetres, and so on. Myriad is a classical Greek name for the number 104 = 10 000, or a group of 10 000 people, etc. ... The English language is a West Germanic language that originates in England. ... A myriagon is a polygon with 10,000 sides. ... Classes Chilopoda - Centipedes Diplopoda - Millipedes Pauropoda - Pauropods Symphyla - Symphylans Four groups of arthropods—the centipedes, millipedes, pauropods, and symphylans—share a number of common features such as a similar body plan consisting of a head followed by an elongate trunk with many legs. ... For the Atari video game, see Centipede (video game). ... This page is about the creature known as the millipede. ...

External links

• Proposal to extend system of units from 10⁻⁶³ to 10⁶³
• BBC article suggesting that a brontobyte is 10²⁷ bytes
• Sybase article suggesting that a brontobyte is 10²⁴ or 10²⁷ bytes (Note article's table a few pages down. Note also it mistakenly places 1 terabyte = 1,000 megabytes. It should be 1 terabyte = 1,000 gigabytes. Also yottabyte is shown incorrectly as zottabyte. With those corrections, it also is clearly 10²⁷.)
• Article suggesting that a brontobyte is 10²¹ bytes
• Article suggesting that a brontobyte was 10¹⁵ bytes before the creation of the prefix peta- (Article is of dubious authority... suggests that Greek letters ν, π, and φ are SI prefixes for nano-, pico-, and femto-, instead of n, p, and f.)
• Article suggesting that brontobyte is 10²⁷ bytes
• Moerner Lab Single-Molecule Research Page (Jokingly defines 1 guacamole = 1 / (Avocado's number) of moles. First archived in 2001 but the definition was up c.1996)