I was wondering if there are differences between the cases of using di- and the cases of using bi-?

For example,

  • why carbon dioxide instead of carbon bioxide?
  • Why binoculars instead of dinoculars?
  • Why bisexual instead of disexual?
  • Why bilateral instead of dilateral?

1 Answer 1


Bi- comes from Latin, Di- from Greek. Which prefix is used would usually depend on the origin of the root of the word.

  • Thanks! Seems like I have to learn history of each such word! Possibly any rule saying that which is used more often in what category? For example, in chemistry, di-; in social science and physics, bi-?
    – Tim
    Commented Feb 9, 2012 at 21:18
  • Precisely. Dioxide is a Greek word, whereas binoculars and bisexual are of Latin origin. +1.
    – Irene
    Commented Feb 9, 2012 at 21:20
  • 1
    Plenty of Latin words have the prefix di- or dis- thought they often lose the sense of 2 by the time they arrive in English: compare bisection and dissection.
    – Henry
    Commented Feb 9, 2012 at 22:51
  • 1
    @Colin: I suspect that all of the "apart" meanings stem from "two". Wiktionary takes di- back to a reconstructed proto-IndoEuropean dwis, and bi- back to dwóh₁, both related to the English twice and two.
    – Henry
    Commented Feb 10, 2012 at 14:48
  • 1
    @Henry. Apparently so. I'm surprised, because "bis" is the normal Latin reflex of "dwis" (cf "bellum" for "duellum"), but the OED agrees that "dis-" is from that same root.
    – Colin Fine
    Commented Feb 12, 2012 at 1:37

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