Inter vs intra: what each prefix means, with examples and a quiz

Inter- means between or among: an interstate highway connects states, and international trade crosses borders. Intra- means within or inside: intrastate travel stays inside one state, and an intravenous drip goes into a vein. To remember which is which, think of intramural sports, played within the walls of one school, and intercollegiate sports, played between schools.

Both prefixes come from Latin, where inter meant “between, among” and intra meant “within.” They often form matched pairs, and swapping one for the other changes the claim. In one hospital, an interdepartmental meeting brings radiology and surgery to the same table, while an intradepartmental meeting is radiology on its own. Ask whether the sentence crosses a boundary or stays inside one.

Inter vs intra at a glance

Inter-: between and among

Inter- is much the more common prefix, and Merriam-Webster lists several shades of meaning. All of them involve two or more separate things:

• Between places or groups: interstate, intercity, international, intercontinental, interstellar
• Shared by two or more: interfaith, interdisciplinary, intergenerational
• Mutual or reciprocal: interrelated, intermarry, interconnect
• Between people: interpersonal, interracial, intercultural
• Between two points in time: interglacial (between ice ages), interwar (between the two world wars)

Many common words, such as interrupt, intervene and interval, came from Latin with inter- already attached. They have no intra- partner, so they never cause trouble.

Intra-: within and inside

Intra- keeps things inside one boundary: one state, one body, one organization, one day. Its senses in Merriam-Webster come down to three:

• Within: intrastate, intramural, intracellular, intradepartmental, intragalactic
• During: intraday, as in intraday trading, where positions open and close within one trading session
• Into, or between layers of, the body: intravenous (into a vein), intramuscular (into a muscle), intradermal (between the layers of the skin)

Intra- words are fewer than inter- words, and they cluster in medicine, biology, statistics, law and business.

Matching inter and intra pairs

Interstate, intrastate and the Constitution

The pair carries legal weight in the United States. The Commerce Clause, in Article I, Section 8, gives Congress the power “To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes.” Commerce among the states is what lawyers call interstate commerce; trade that begins and ends inside one state is intrastate. Names don't always follow the prefix, though: Hawaii's Interstates H-1, H-2 and H-3 all run on Oahu, because “Interstate” there names the national highway system, not the route.

Intramural, intercollegiate and intermural

Intramural means literally “within the walls.” In its school sense, it means competition only within the student body: intramural soccer is students of one college playing each other. Games against other schools are intercollegiate, or interscholastic for high schools. You may see intermural, but Merriam-Webster defines it only as “lying between walls”; it isn't a sports term.

Research funding uses the same split. The National Institutes of Health calls the work in its own laboratories intramural research and the grants it makes to universities and other institutions extramural research.

Not to be confused: intro- and extra-

Two other prefixes get mixed up with intra-. Intro- means into or inward, as in introduce and introspection, which is why introvert is spelled with an o, not an a. Extra- means outside or beyond, as in extracurricular and extrajudicial, and extramural is the true opposite of intramural.

Hyphenation: interstate or inter-state?

Hyphens don't change what either prefix means; they are a spelling convention, and house styles vary. Dictionaries close most of these words: interstate, intrastate, intramural, intraday, even where a consonant doubles at the join, as in interrelated and interracial. You'll usually see a hyphen in three places:

• Before a capital letter: inter-American, as Merriam-Webster spells it, or intra-EU trade.
• Where two vowels meet: intra-arterial, as Merriam-Webster spells it, keeps the two a's apart.
• When the prefix stands alone in a pair: inter- and intrastate flights, intra- and extracellular fluid. The hanging hyphen tells the reader the rest of the word is coming.

Beyond those, fields and publishers set their own habits, so you'll see both inter-rater and interrater in research papers. Follow your journal's or publisher's style, record the choice on your style sheet, and keep it for the whole document. For the difference between a hyphen and the longer dashes, see em dash vs en dash.

Inter and intra as words

Inter is also a verb with a different origin: to inter someone is to bury them. It came into English in the 14th century, through French, from the Latin in plus terra, “earth.” Its noun is interment, a burial, which is not internment, the confinement of people, often in wartime. A cemetery records interments; a history book describes internment camps.

Intra has no life as an English word on its own, but both forms appear in Latin phrases common in legal and academic writing: inter alia, “among other things”; intra vires, “within the powers,” and its opposite, ultra vires, “beyond the powers.” A council that sells a park it had no authority to sell has acted ultra vires.

Inter and intra in sentences

1. The bill would ban the interstate sale of untested batteries.
2. Most of the airline's routes are intrastate: they never leave California.
3. She played intramural volleyball all four years.
4. Our intercollegiate debate team travels to six tournaments a year.
5. The drug is given intravenously over two hours.
6. Interpersonal conflict was the main reason people left the team.
7. Intrapersonal skills, such as managing your own stress, are harder to teach.
8. The holiday calendar and the expense forms are on the intranet.
9. Intercompany loans between the parent and its subsidiaries must be documented.
10. Interfaith services were held in the town square.
11. The two coders showed high inter-rater reliability.
12. Intra-rater reliability asks whether one coder scores the same material the same way twice.
13. Intracellular calcium rose within seconds.
14. The intercity bus takes four hours.
15. Intraday prices swung by 6 percent before the close.
16. Interdepartmental meetings are on Mondays; intradepartmental ones happen whenever the chair calls them.
17. Radio grew fast in the interwar years.
18. Intraspecific competition is fiercest when food is scarce.

Quiz: inter- or intra-?

Add the right prefix to each blank, then check the answers.

1. A trucking firm that only operates inside Ohio does ____state business.
2. A league in which students of one university play each other is ____mural.
3. A conference for chaplains of several religions is ____faith.
4. A shot given into a muscle is ____muscular.
5. A treaty signed by many countries is ____national.
6. Email between the marketing and legal departments is ____departmental.
7. A private network for a company's staff is an ____net.
8. Fluid inside the cells is ____cellular.
9. Travel from one star system to another is ____stellar.
10. Price moves within a single trading day are ____day.

Check prefixes across a document

In a report, a grant proposal or a thesis chapter, an inter- that should be intra- changes the claim, and both look right at a glance. Read each one against the boundary the sentence means: crossing it, or staying inside it.

In a long paper, you can ask the Strokes Assistant to review the document for inconsistent hyphenation, so that inter-rater on page 4 and interrater on page 19 get settled one way.

The same check suits other look-alikes that change a claim, such as further and farther, and short words that sound alike, such as to and too.