Vocab activities for your classroom

Getting "In" to Prefixes

Flexible and inflexible are opposites, but flammable and inflammable are not. Why is this? From a morphological and contextual perspective, Susan Ebbers discusses how to help students come to grips with confusing words, including inflammable, impregnable, and infamous.

One day, combustible cans were "inflammable" and the next, "flammable" — or so it seemed. In fact, the transition from inflammable to flammable took several decades. If you consult some sources, inflammable is still in transition. According to Grammarist, "inflammable is still common on product labels and appears from time to time in edited publications, but it fell out of favor around 1970." Today, we still refer to "inflammatory speeches" and "inflammations of the skin" so flammable did not fully replace inflammable.

The word inflammable comes from inflame. The root FLAM denotes 'to kindle, to set on fire, to burn.' This word came to English via Latin by way of French. It is also related to inflame, inflaming, and the French loan words flambeau, flambé and flamboyant.

Misinterpretation of the word occurred because of the prefix in-. This prefix comes in several forms: in-, im-, il-, and ir-. No matter the guise, it has two main interpretations or senses, plus a possible additional sense:

  1. Prefix in- denotes 'not' or 'lacking' as in inedible, illegal, implausible, irreverent.
  2. Prefix in- denotes 'into, in, on, upon' as with income, intake, implant.
    a. Sometimes the prefix ALSO serves as an intensifier, much like we use very, as perhaps with incandescent, "to glow from within to an intense degree."

So, inflammable could potentially be interpreted three ways:

  1. 'not flammable'
  2. 'flammable from within'
    a. 'extremely flammable from within'

The second and perhaps even the third options are correct; inflammable indicates that something is "flammable from within, able to burst into flame, able to be set on fire." See Online Etymology Dictionary.

Understandably, many folks thought inflammable meant 'not flammable' — hence the decision to change the danger signs. Note that the related derivations retain the prefix in-, as with inflammatory, inflammation, inflammability.

Because inflammable derives from Latin, it is likely to have some look-alikes or "cognates" across the Romance languages. But when it changed to flammable in English, did it also change in Italian? Perhaps not. See the Italian sign to the left.

To reduce confusion, perhaps in this case we should omit words altogether and communicate through pictures. This involves semiotics, the study of how signs, symbols, and icons convey meaning.

Implications for Teaching & Standards:

I. Help students apply this lesson to two similar word pairs:

1) This same prefix is partially responsible for the confusion caused by impregnable and impregnate. Ask students, "How do lessons learned from inflammable apply to impregnate and impregnable?" Provide some context: "The fort is impregnable" versus "We must impregnate the polar bear." Knowing the two meanings of the prefix will help students, especially when the surrounding context is clear. In the VT word map below, impregnable most commonly denotes 'secure' but it also means 'conceptive, able to become pregnant.' In this case, the prefix in- denotes 'in, within' and is spelled in one of its alternate forms, im-.

2) "What about infamous versus famous?" Hmm… it should help to consider the word infamy and the expression "went down in infamy" (not a happening place!). The word fame comes to us from Old French fame and from Latin fama, where it meant "reputation" and "much talked about" so a person who lacks a good reputation is considered infamous — 'not reputable.'

Fame (and famous) came to us through mythology, much like we got "Pandora's box" and "the Midas touch." In Roman mythology, the goddess Fama was the personification of rumor. Sounds infamous, and she is not a well-known deity, despite her big mouth. Try to think of more words from the myths. Think of the name of a famous ship — her captain went down in infamy. See more mythology-based words and phrases at The Hellenic Times.

Suggested Extension Activity:

Working with peers at home or at school, have students try the sorting activity in the worksheet here. Discuss what the prefix "means" in each bold word. If you are not sure, type the word into the Online Etymology Dictionary:

Word in context

Meaning of the prefix in- (im-, il-, ir-)


"not" or "lacking"

"in, on, within"


  • The car's intake valve is clogged.




  • This old fruit is inedible.




  • Gasoline is inflammable.




  • Is the fort truly impregnable?




  • Is the fatherless child illegitimate?




  • Their odd actions are illogical.




  • What a bright, incandescent light!




  • The bells chime at irregular times.




  • The patient received a heart implant.




  • Inject some medicine into the cat.




  • What an incredible book!



cred is Latin for 'to believe'

  • This handwriting is illegible




  • Our dear pet is invaluable to us.



[so treasured, beyond value,  priceless]

  • That wicked fellow is immoral.




  • What strange, irrational behavior!




  • We paid our income tax.




  • Have the insight to just walk away.




  • Let's insert the pearl here.




Note: These ideas align with the Common Core State Standards for English Language Arts. Here is an example from grade 5:

COMMON CORE STATE STANDARD: Vocabulary Acquisition and Use L.5.4.  

Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade 5 reading and content, choosing flexibly from a range of strategies.

  • Use context (e.g., cause/effect relationships and comparisons in text) as a clue to the meaning of a word or phrase. 
  • Use common, grade-appropriate Greek and Latin affixes and roots as clues to the meaning of a word (e.g., photograph, photosynthesis). 
  • Consult reference materials (e.g., dictionaries, glossaries, thesauruses), both print and digital, to find the pronunciation and determine or clarify the precise meaning of key words and phrases.

Susan Ebbers is the creator of Vocabulogic, an edublog focusing on word knowledge and linguistic insight. She is a former K-8 teacher and principal, a Cambium Learning curriculum author, a national literacy consultant, and a doctoral candidate. Her research interests pertain to word-learning aptitude, measurement design, and motivation theory. In her spare time she writes poetry, including the Jamie's Journey series of children’s books. Visit her website or follow her on Facebook. To read more about how morphological insight contributes to vocabulary growth, read her research overview on Vocabulogic.

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