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Resolution 4 of the CGPM (1991)
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of the CGPM at its 19th meeting (1991)

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SI prefixes zetta, zepto, yotta and yocto* (3)  

The 19th Conférence Générale des Poids et Mesures (CGPM),

decides to add to the list of SI prefixes to be used for multiples and submultiples of units, adopted by the 11th CGPM, Resolution 12, paragraph 3, the 12th CGPM, Resolution 8, and the 15th CGPM, Resolution 10, the following prefixes:

Multiplying factor Prefix Symbol
1021 zetta Z
10–21 zepto z
1024 yotta Y
10–24 yocto y

Comptes Rendus de la 19e CGPM (1991), 1992, 185
Metrologia, 1992, 29(1), 1-7


The names zepto and zetta are derived from septo suggesting the number seven (the seventh power of 103) and the letter "z" is substituted for the letter "s" to avoid the duplicate use of the letter "s" as a symbol. The names yocto and yotta are derived from octo, suggesting the number eight (the eight power of 103); the letter "y" is added to avoid the use of the letter "o" as a symbol because it may be confused with the number zero.

The reader should note that the official version of this Resolution is the French text.