Quick Summary

The English prefixes bi-, derived from Latin, and its Greek variant di- both mean “two.” The Latin prefix is far more prevalent in common words, such as bilingual, biceps, and biped; the more technical Greek di- appears in such words as diphthong and dilemma. You can remember bi- easily by thinking of bicycle, which has “two” wheels, and di- by remembering that the “dioxide” of carbon dioxide means that there are “two” oxygen atoms in the molecule CO2.

Double with Bi- and Di-!

The English prefixes bi-, derived from Latin, and its Greek counterpart di- both mean “two.” These “two” prefixes are used commonly, especially bi-.

Let’s begin with the frequently used English prefix bi-, which means “two.” A bicycle has “two” wheels. A biped, like homo sapiens, walks on only “two” feet, perfect for a bicycle! A biped also has a primary muscle known as a biceps, which has “two” heads that attach to bones in the upper arm. A biped also has binocular vision, which mean that she uses “two” eyes to see. And does a biped who is bilingual have “two” tongues? No … it just means that he speaks “two” languages fluently.

Calendars have many events that use the prefix bi-, which means “two.” For instance, the bicentennial of an institution or nation is a celebration commemorating “two”-hundred years of existence. Two words that are easily confused are biannual and biennial. A biannual event happens “two” times every year, whereas a biennial celebration happens once every “two” years. You can remember the difference because alphabetically the “a” of biannual comes before the “e” of biennial, and also happens first in time.

Now let’s go on to the Greek variant of bi-, or di-, which also means “two.” Carbon dioxide, or CO2, is a gas whose molecule has “two” oxygen atoms. If you are in a dilemma, you are unable to choose between “two” options, often both unappealing! A diphthong is a pair of or “two” vowels in a row in a word in the same syllable, such as “cloud” and “foil.” And a dipterous insect? It simply has “two” wings!

Today you got “two” for your money with both bi- and di-. Now you no longer need to be “twice” shy with words such as bipartite and diphyllous, for you know that each has something “to” do with “two!”

  1. bicycle: vehicle with ‘two’ wheels
  2. biped: animal that walks on ‘two’ feet
  3. biceps: muscle with ‘two’ heads that attach to bone
  4. binocular: using ‘two’ eyes to see
  5. bilingual: of knowing ‘two’ languages fluently
  6. bicentennial: of a ‘two’ hundred year anniversary
  7. biannual: of happening ‘two’ times each year
  8. biennial: of happening once every ‘two’ years
  9. dioxide: having ‘two’ oxygen atoms
  10. dilemma: a situation with ‘two’ equal choices
  11. diphthong: ‘two’ vowels as one syllable
  12. dipterous: of ‘two’ wings
  13. bipartite: having ‘two’ parts
  14. diphyllous: having ‘two’ leaves


  • bifurcate

    When something is bifurcated, such as a path, it is divided into two separate parts that fork off from each other.

  • combine

    When two or more things combine, they join or come together.

  • biannual

    occurring or payable twice each year

  • bicentenary

    of or relating to or completing a period of 200 years

  • bicentennial

    of or relating to or completing a period of 200 years

  • biceps

    any skeletal muscle having two origins (but especially the muscle that flexes the forearm)

  • bicycle

    ride a bicycle

  • biennial

    having a life cycle lasting two seasons

  • bifocals

    eyeglasses having two focal lengths, one for near vision and the other for far vision

  • bilateral

    having identical parts on each side of an axis

  • bilingual

    using or knowing two languages

  • billion

    denoting a quantity consisting of one thousand million items or units in the United States

  • binoculars

    an optical instrument designed for simultaneous use by both eyes

  • binomial

    of or relating to or consisting of two terms

  • bipartite

    divided into two portions almost to the base

  • biped

    having two feet

  • biracial

    consisting of or combining two races

  • bisect

    cut in half or cut in two

  • bisection

    dividing into two equal parts

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