Morphological composition determines how morphemes combine to form words. Affixes attach in prescribed orders (prefix-stem-suffix in English) and may alter stems (ablaut, reduplication). Some morphemes are productive (freely attach to new bases); others are lexically restricted. The compositionality principle holds that word meaning should be predictable from parts—when it is not, the word has special meaning (idiomaticity) that must be stored lexically.
Analyze morphologically complex words, decomposing them into morphemes and predicting composition. Examine which affixes are productive by testing them on nonce words, and explain idioms as composition failures.
You already have a solid grasp of the basic building blocks from inflectional and derivational morphology: inflectional morphology adds grammatical information (tense, number, case) without changing a word's category or core meaning, while derivational morphology creates new words by changing category or meaning. Morphological composition is the broader study of how all these processes — and more — interact to build complex words, and why the results are sometimes predictable and sometimes not.
The compositionality principle is the central guiding assumption: the meaning of a complex word should be derivable from the meanings of its parts plus the rules for combining them. *Unhappiness* = un- + happy + -ness; you can recover the meaning by applying each morpheme's contribution in order. This is composition working as expected. But language is full of cases where composition breaks down — and the pattern of breakdown is itself informative. *Blackbird* is composed of *black* + *bird*, but a blackbird is a specific species, not just any bird that happens to be black. *Understand* contains *under* and *stand*, but its meaning is opaque to etymological decomposition in modern English. These are lexically stored idioms — the morphological structure is present, but the meaning is not compositionally derived from it; instead, the whole item is stored as a unit in the mental lexicon.
The concept of morphological productivity is where composition meets competence. A morpheme is productive if native speakers freely apply it to new bases to create new words, including words they've never heard before. The suffix *-ness* is highly productive in English: *unhappiness*, *blueness*, *snarkiness*, *Fridayness* are all well-formed. The suffix *-th* (as in *warmth*, *depth*, *growth*) attaches only to a historically frozen set of bases — you cannot say *\*coldth* or *\*greentn*. Testing productivity by applying morphemes to nonce words (made-up words) reveals the underlying generative rules: if speakers accept *wug → wugs* but reject *\*wug → wugth*, they are unconsciously tracking the productivity of the morpheme.
Affix ordering is the compositional constraint that most surprises learners. Affixes in English attach in a fixed sequence: the derivational morphology typically applies first, then inflectional morphology applies last. You get *un-happy-ness* (derivational layers, inside-to-outside), but not *\*happy-ness-un*. Within the derivational layer, ordering is further constrained by the base's category: *-ize* attaches to nouns and adjectives to make verbs (*legalize*, *computerize*), but *-ation* then attaches to *-ize* verbs to make nouns (*legalization*). Violate the order and the result is ungrammatical. The skill of morphological analysis is reconstructing this order from the surface form — tracing the derivational tree that produced the word — and identifying where meaning is compositional and where it must be stipulated.