Hepta- was an old prefix proposed a few years before zetta-.[1] It comes from the Greek word meaning 7, as it was the 7th power of 1000. It was later removed and replaced with zetta- beacuse its symbol h was already used as a symbol for hecto-.

The reciprocal of this prefix is septo- from Latin septem meaning 7. It was also dropped and replaced with zepto- because its symbol s was also already used as a symbol.[2]

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