Mono means “sole, only”, proto means “first”, hen means “one."

Duo means “two”, di(s) means “twice."

Hepta means “seven” or “seven times”, hebdoma means “seventh."  A hebdomadal event occurs every seventh day, or once a week.

"Nona”, meaning “ninth”, is from Latin “nonus”, not from Greek.

"Deci”, meaning “a tenth”, is from Latin “decimus”, not from Greek.

Generally, we use Greek prefixes when we are multiplying, so decathlon means "ten contests" and a kilometer equals 1,000 meters.

We tend to use Latin prefixes when we divide, so a millimeter is a thousandth of a meter.

An exception is the prefix "nano," which comes from the Greek word for "dwarf."

Mega, usually meaning "million," is from the Greek word for "large."

Tera, the next step up in size, is from the Greek word for "monster."

Pico, denoting a factor of 10^-12, is from the Spanish meaning "a little bit."

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