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Morpheme #3 of 14Brown's Stage II27–30 months

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.

At a glance

Acquisition order
#3 of 14 (Brown 1973)
Symbol / form
on
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

Brown tracked "on" as the second of two locative prepositions in his 14, paired with "in". "On" encodes a surface-contact spatial relation rather than containment ("book ON table", not "book IN table"). It is acquired in the same window as "in" — usually within weeks of each other in the same child — which gave Brown his evidence that the two prepositions are not learned as independent lexical items but as a small grammatical system encoding the basic spatial relations of containment and support. Like "in", it appears as a free morpheme in adult English and there is no inflectional suffix involved in scoring it.

Acquisition trajectory

The trajectory mirrors "in" almost exactly. First productive uses are in high-frequency surface-contact routines that the family talks about every day: "on table", "on chair", "on bed", "on the floor". Children often produce "on" first in fixed phrases ("on the floor!" as a request to be put down) before extending it to novel surface-contact contexts. Brown noted that errors of substitution between "in" and "on" are rare in typical development — once a child distinguishes containment from support spatially, the lexical mapping is reliable. A small group of children acquire "on" a month or two before "in" or vice versa; that small ordering difference is unremarkable and does not suggest a problem. The locative pair reaches 90% mastery by approximately 30 months in typical development, in late Stage II.

Examples in obligatory context

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

  • “Book on table.”
  • “Hat on head.”
  • “Cup on chair.”
  • “Sit on couch.”
  • “Cat on bed.”
  • “Put it on top.”

Common errors during the acquisition window

  • Substitution with "in": "book in table" for "book on table"
  • Omission of the preposition: "book table" for "book on table"
  • Use of a generic spatial term like "there" with a pointing gesture
  • Persistent contact-context omission in connected play even after isolated production

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

Because the two locative prepositions reach mastery in the same window, clinicians should score them together and report a combined obligatory-context percentage in the language sample. A child who consistently omits both "in" and "on" at 30 months while producing multi-word utterances is showing a grammatical lag worth following with a structured probe and a return visit. By contrast, isolated substitution of "in" for "on" or vice versa in an otherwise grammatically intact sample is unremarkable and reflects normal lexical sorting in late Stage II. The two locative prepositions are early enough in Brown's order that their delay points back to the broader closed-class system rather than to a specific spatial-cognition deficit, and a full language sample is the right next step rather than a single-item probe.

“Score "in" and "on" as one item in your report, not two. Children acquire them as a pair, lose them as a pair, and a single percentage tells the IEP team everything they need.”
Score the locative pair together

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

#2 · Stage II

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".

#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. Bowerman, M., & Choi, S. (2001). Shaping meanings for language: Universal and language-specific in the acquisition of spatial semantic categories. Language Acquisition and Conceptual Development, 3, 475–511.