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Morpheme #2 of 14Brown's Stage II27–30 months

Preposition "in"

The locative preposition "in" is the second of Brown's 14 morphemes, typically mastered between 27 and 30 months alongside its sibling "on".

At a glance

Acquisition order
#2 of 14 (Brown 1973)
Symbol / form
in
Brown's stage
II
Typical age of mastery
27–30 months

Mastery is defined as correct use in 90% of obligatory contexts across three consecutive language samples (Brown 1973). Individual variation is wide — these ranges describe typically developing English-speaking children and should be cross-referenced against the primary literature before clinical use.

What this morpheme is

Although "in" is a free morpheme rather than a bound suffix, Brown included it in his 14 because it functions grammatically — it conveys spatial containment and acts as a locative marker that obligates a particular grammatical frame ("ball IN box", not "ball box"). It is one of two locative prepositions Brown tracked, the other being "on", and the two are nearly always acquired in the same window. "In" is interesting developmentally because the spatial concept it encodes is usually understood months before the word itself appears in production — comprehension precedes production by a wide margin.

Acquisition trajectory

Toddlers begin using "in" in early Stage II and reach 90% obligatory-context use by approximately 30 months in typical development. The first productive contexts are usually high-frequency containment routines that the family talks about every day: "in box", "in cup", "in bed", "in car". Brown noted that there is essentially no overgeneralisation phase for "in" — it is used in strictly containment contexts from the first instance and rarely substituted into non-containment frames. The acquisition of "in" runs in lockstep with the acquisition of "on", and both prepositions typically reach criterion within a few weeks of each other in the same child. When delays appear, they tend to affect both prepositions together rather than splitting between them. Bilingual Spanish-English children may use the all-purpose Spanish "en" frame and produce English "in" in contexts where adult English would use "on", which is a normal cross-linguistic interaction and not a deficit.

Examples in obligatory context

These are the kinds of child utterances a clinician would code as a correct production of in when scoring a language sample.

  • “Ball in box.”
  • “Doggie in car.”
  • “Baby in bed.”
  • “Cookie in cup.”
  • “Put it in there.”
  • “Mommy in kitchen.”

Common errors during the acquisition window

  • Omission of the preposition entirely: "ball box" for "ball in box"
  • Substitution with "on" in a containment context: "ball on box"
  • Use of a non-locative substitute like "go" or pointing in early Stage II
  • Persistent dropping in connected speech even after isolated naming is correct

Many of these errors are developmentally normal during the acquisition window. The clinical signal is persistence past the typical age of mastery, not the presence of any single error in early production.

Clinical relevance and scoring

A toddler who is producing two- and three-word combinations at 30 months but still omits "in" and "on" in obligatory containment contexts is a candidate for a closer look at expressive language. The two locative prepositions are early enough that their absence in a child who is otherwise stringing words together signals a grammatical lag rather than a vocabulary gap. To score "in" reliably, code every utterance in a 50-utterance language sample where the adult interpretation requires a containment preposition; if the child omits the word entirely, mark it as an error in obligatory context. Clinical caveat: do not penalise utterances where the child substitutes a deictic gesture or "there" for the preposition unless the child has clearly been observed producing "in" elsewhere in the sample, because pointing is a developmentally appropriate alternative in very early Stage II.

“When a 30-month-old omits "in" and "on" in connected speech, the gap is rarely about prepositions — it is about the whole closed-class system. Sample five minutes of play and watch what else is missing.”
Closed class is a closed system

Free tools for scoring this morpheme

MLU Calculator

Paste a language sample and get Mean Length of Utterance in morphemes and words, total utterances, total morphemes, and the matching Brown's stage. Implements Brown (1973) morpheme counting rules and runs entirely in your browser.

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Brown's Stages Lookup

Interactive reference for Brown's (1973) five stages of grammatical development. Look up a stage by child age or MLU, see the MLU range, acquired morphemes, example utterances, and clinical milestones for each stage.

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PGU Calculator

Paste a language sample, tap G or U per utterance, and get Percent Grammatical Utterances (PGU) live. Returns the Eisenberg & Guo (2013) severity band (typical / borderline / clinical concern) once at least 25 utterances are scored. Mobile-friendly scoring table, client-side, no sign-up.

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Related morphemes

#3 · Stage II

Preposition "on"

The locative preposition "on" is the third of Brown's 14 morphemes, mastered alongside "in" between 27 and 30 months in typical development.

#1 · Stage II

Present Progressive -ing

Present progressive -ing is the first of Brown's 14 grammatical morphemes that English-speaking toddlers master, typically between 19 and 28 months of age.

#4 · Stage II

Regular Plural -s

Regular plural -s is the fourth of Brown's 14 morphemes, marking countable noun plurality and reaching mastery between 27 and 33 months.

References

  1. Brown, R. (1973). A First Language: The Early Stages. Harvard University Press, pp. 322–337.
  2. de Villiers, J. G., & de Villiers, P. A. (1973). A cross-sectional study of the acquisition of grammatical morphemes in child speech. Journal of Psycholinguistic Research, 2(3), 267–278.
  3. Tomasello, M. (1987). Learning to use prepositions: A case study. Journal of Child Language, 14(1), 79–98.