Semi-, Demi-, and Hemi-

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A reader asks:

Is there any rule for when to use ‘semi’ and when to use ‘half’?

The Latin prefix semi– means “half.”

The earliest “semi-” words documented in English are semicircular (1432-1450 and semi-mature (c.1440). Both William Langland (c.1332-c.1386) and Chaucer (c.1343-1400) use “semi-” constructions. According the OED,

In the 16th-18th c., the number of permanent compounds was increased mainly by the accession of terms more or less technical [such as semicircle and semivowel].”

Although there’s no rule, sometimes half would be the better choice, as in this example from Fowler:

This would be an immense gain over the existing fashion of a multitude of churches ill-manned & semi-filled.

Some common semi- words:

Two other prefixes that mean “half” are hemi,-, from the Greek, and demi-, from Old French.

In some English words, the French prefix is attached to an English word, as in demigod. Sometimes, as with demitasse and demimonde, the entire word is French.

demimonde: n. The class of women of doubtful reputation and social standing, upon the outskirts of ‘society.’[Fr.; lit. ‘half-world’, ‘half-and-half society’, a phrase invented by Dumas the younger.]

demitasse: n. a small coffee cup. [Fr., lit. ‘half-cup’]

The prefix hemi– is not as common as the other two. The most familiar word is hemisphere. Less common are words like hemicycle, hemistich, and hemitrope.

In some contexts, the prefixes are used as nouns.

Men in U.S. truck commercials rapturize over “Hemi” trucks. This use of hemi refers to a type of internal combustion engine that has a “bowl-shaped or ‘hemispherical’ combustion chamber.”

In England, a semi [sĕm’ē] is a semi-detached house. In the U. S., a semi [sĕm’ī] is a big truck of the tractor-trailer variety.

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9 thoughts on “Semi-, Demi-, and Hemi-”

  1. It would seem that

    Semi implies cyclical or timing (eg., semi-coupon payments)


    half implies volume (eg. half-filled glass)


  2. Haven’t had time to really digest this whole post, so will only address the idea of “semi.”
    “Semi” seems to have taken on the meaning of “pseudo,” as in, “I have to go to this semi-important meeting.”
    Also, it seems to me that people use it to mean “approximately” or “somewhat,” when it’s used to speak of things that can’t readily be measured. So if you can see that a glass is exactly half full/empty, you would use the word “half.” However, can you measure if someone is exactly “half” conscious? Not really (OK, aside from the Glasgow coma scale score, but even that is based on an odd number [15], and nobody ever scores a 7.5!) Basically, you can’t exactly measure abstract things (something can’t be half-important or half-attractive), so you say “semi.” It then takes on the meaning of “somewhat,” and, depending on context and tone of voice, can be interpreted differently.

  3. You’re forgetting possibly the only place where semi, demi and hemo are all used together…


    You must remember music at school, we all did it…
    Semibreves, minims, crotchets, quavers, semi-quavers, demisemiquavers and hemidemisemiquavers…

  4. “The earliest “semi-” words documented in English are semicircular (1432-1450 and semi-mature (c.1440). Both William Langland (c.1332-c.1386) and Chaucer (c.1343-1400) use “semi-” constructions.”

    I’m having trouble reconciling the dates in those two sentences, as Langland and Chaucer both appear to have died before the first “semi-” words were documented. Is this something about HOW they used semi-? I notice you refer to “constructions” rather than “words” for the two of them. . .

  5. @Kathryn,
    I suppose I should have tried to make my thought clearer.

    The “semi-” constructions in Langland and Chaucer that I was thinking of are words that haven’t had the staying power of semicircular, for example, semivif and semi-soun.

  6. Maeve–thanks. I thought it might be something like that. I am regrettably one of those people who cannot, when presented with two or more numbers which are in some way related, refrain from examining their relationship. I have the WORST time with fictionists who mention specific years for past events, and don’t seem to have checked to see that the gap between past and present actually works (“how can the protagonist be still in college if he actually remembers the invasion of Grenada?”). Your explanation clears it all up splendidly, so thanks.

  7. Sometimes, even in mathematics, “semi” does not mean exactly one-half. In abstract algebra, there is a vital kind of a set that is called a “group”, with one mathematical operation and about eight well-defined properties.
    Then there is the “semigroup”, which is a set that is missing a couple of the properties of a group (not missing half of the properties).
    I had studied group theory, but I had never heard of a semigroup until I was reading an article about the mathematician Andrew Wiles. Dr. Wiles used the properties of semigroups (among 101 other items) in his proof of Fermat’s Last Theorem.
    Of course, his proof uses at least 200 pages of densely-typed text. I have not heard anything about the proverbial 20-page proof that we have been waiting for!

  8. Most of the words using the prefix “hemi” seem to be scientific words. The obvious one is “hemisphere”. “Hemitropic” is a word from crystallography: the study of the structure of crystals — and there is an amazing variety of different kinds of these.
    “Hemihydrate” is a word from chemistry:
    a hydrate (such as plaster of Paris) containing one-half mole of water to one mole of the other substance in the hydrate.
    Then there is the biological term “hemipenis”. Most male snakes have one hemipenis on the left and one on the right. That way, from either way that he approaches a female, he can mate with her!

    : a hydrate (such as plaster of paris) containing half a mole of water to one mole of the compound forming the hydrate

  9. Some common “semi-” words: {semiabstract, semiannual, semiautomatic, semicolon, semiconscious}

    Also: semimonthly, SEMIFINAL (not hyphenated!), semihuman or semi-human, semi-independent (like Greenland, Aruba, Curacao, and Kosovo), and semipermanent. Also, in astronomy and math, the “semimajor axis” has one-half the length of the major axis of an ellipse.

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