Morphology and Word Formation › Morphemes: The Atoms of Word Structure
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Morphology is the study of the internal structure of words. Its central unit is the morpheme.
A morpheme is the smallest meaning-bearing unit of a language. It may be a whole word (like dog) or a piece of one (like the plural -s or the negative prefix un-). Crucially, "smallest" refers to meaning, not size: cat has three characters but is a single morpheme, while cats has four characters but contains two.
Every word form can be analyzed as a structured combination of morphemes. Linguists distinguish a few roles these morphemes can play:
- Root: the core morpheme carrying the word's primary lexical meaning. In unhappiness, the root is happy. Every simple (non-compound) word has exactly one root, and it is usually the piece that survives when all decoration is stripped away; compounds, covered below, combine two or more roots. Roots may themselves be free (like happy, which stands alone as a word) or bound (like the Latin-derived root -ject in project and inject, which cannot appear without an affix).
- Affix: a bound morpheme attached to a root. Prefixes attach before (un-, re-, dis-), suffixes after (-ness, -ed, -ly), infixes inside (rare in English, common in Tagalog), and circumfixes surround the root (German ge-...-t in past participles like gemacht). Affixes are the workhorses of morphology: they carry grammatical information like tense and number, or shift meaning and category as derivation does.
- Stem: the form a morphological operation attaches to. In unlockable, the stem for -able is the entire derived word unlock, not just the root lock. The distinction between root and stem matters because affixes do not always attach directly to roots; they often attach to already-built structures, which is why word formation is best understood as a sequence of operations rather than a flat pile of pieces.
A lexeme is an abstract vocabulary item: the unit listed in a mental dictionary. Run, runs, ran, and running are all word forms of a single lexeme, RUN. A lemma is the canonical citation form chosen to represent that lexeme, conventionally the infinitive for verbs (run) and the singular for nouns (cat). Keep these distinct from the surface token, which is what appears in text. One lemma corresponds to many tokens.
Morphemes come in two flavors. Free morphemes can stand alone as words (book, green, dog). Bound morphemes cannot and must attach to something else (-s, un-, -able). English is relatively light on bound morphology, but even here the distinction shapes everything from plural handling to adjective comparison. In many other languages the balance tips heavily toward bound material, and we will see shortly how that imbalance reshapes entire NLP pipelines.
Not every decomposition a computer proposes is linguistically real. Carpet contains the letter sequence car-pet, but neither piece contributes meaning; the word is a single monomorphemic borrowing. Similarly, relish has no semantic relation to re- + lish. Morphology is about meaning-bearing units, and this constraint is what separates genuine morphological analysis from naive string matching. The test is simple to state and hard to automate: does each proposed piece carry a consistent meaning across the words it appears in? Un- means "not" or "reverse" in thousands of English words, so it is a real morpheme; car- means nothing systematic, so it is not. This gap between string similarity and semantic reality will resurface repeatedly, both in our rule-based segmenter later in this chapter and in the statistical tokenizers of Part V.