Morphemes: the parts of words

The word cats has four letters but only two units of meaning: cat, the animal, and -s, the signal that there is more than one. The word unhappiness has three: un- for negation, happy for the core idea, and -ness to turn the adjective into a noun. Each of those units is a Morpheme: the smallest unit that carries meaning or a grammatical function. Jurafsky and Martin put it in one line: a morpheme is a minimal meaning-bearing unit in a language, so fox is one morpheme while cats is two (SLP3, section 2.2).

Try the segmentation before the theory. In SLP3 the sentence Doc worked carefully washing the glasses is cut as Doc work-ed care-ful-ly wash-ing the glass-es. Every hyphen marks a boundary where one meaning stops and another begins. Doc and the are single morphemes; carefully is three. Notice what the cuts are not: they are not syllables, and they are not places where the spelling changes. They are places where meaning changes.

Free and bound

The first sorting of morphemes is by independence. A free morpheme can occur by itself as a word: cat, walk, happy. A bound morpheme never occurs alone and is always attached to some other morpheme, the way -s in dogs is (SIL Glossary of Linguistic Terms). Bound morphemes are the affixes: every un-, -ed, -ing and -ness you will ever segment. The test is simple. Could you say it on its own and be understood? happy, yes. ness, no.

Roots, stems and affixes

The second sorting is by job. SLP3 divides the morphemes of a word into roots, the central morpheme supplying the main meaning, and affixes, which add meanings of various kinds: in worked, work is the root and -ed the affix; in glasses, glass is the root and -es the affix (SLP3, section 2.2). The root is the core lexical material, and in English it is usually a free morpheme.

The lecture adds a third word that students often blur into root: the Stem. A stem is the inflectable base, the thing the grammatical endings attach to. In a simple word like walk the root and the stem are the same string, which is why one slide writes them as root/stem. They come apart once a derived word is inflected. In unhappinesses the root is happy, the core of the meaning, but the plural -es attaches to the whole stem unhappiness. The root is what the word is about; the stem is what the grammar sees.

Affixes are then named by where they sit. A prefix goes before the stem (un-). A suffix goes after it (-ed, -ing). An infix is inserted inside the stem, and a circumfix wraps around it in two parts at once. Both are rare or absent in English. The standard textbook circumfix is the German past participle, as in ge-spiel-t, played, where ge- and -t together express one meaning (Haspelmath and Sims, chapter 2), and the usual English illustration of infixation is the jokey abso-bloody-lutely. Keep these two as illustrations; the slide only says they exist and are rare. Compounding is different again: it joins two roots without any affix, so snowman is snow plus man and laptop is lap plus top, two free morphemes in one word.

Arabic roots: a skeleton, not a string

Everything above assumed the root is a contiguous piece of the word. Arabic breaks that assumption, and it is the reason this part matters so much for your work. The Arabic root for writing is K-T-B, the three consonants ك ت ب. Those three letters never appear next to each other as a word. Instead they are slots, and a pattern of vowels and sometimes extra consonants is threaded between them. The same root with three patterns gives kataba كَتَبَ he wrote, kitāb كِتَاب book, and kātib كَاتِب writer. Look at the Arabic script, where the radicals are highlighted: ك, ت, ب keep their order in every form, and only the short-vowel marks and the long vowel ا between them change.

This is Templatic morphology, also called root-and-pattern morphology. The root is an abstract consonantal skeleton, and actual words are formed by adding the vowels and non-root consonants that go with a particular morphological category around the root consonants (Wikipedia, Semitic root). Ryding's reference grammar treats the root and pattern as the two fundamental building blocks of an Arabic word: the root supplies the semantic field, the pattern supplies the grammatical and derivational shape (Ryding, chapter 3). You will see the mechanism itself in the third concept of this part; for now the point is that K-T-B is a root in exactly the sense defined above, core lexical material, but it is discontinuous.

Why NLP cares

SLP3 names three candidates for what a Token should be: words, morphemes and characters (SLP3, section 2.4). Earlier it observes that because morphemes are hard to define consistently across languages, it is very difficult to use morphemes as a standard for Tokenization cross-lingually (SLP3, section 2.2). That difficulty is the whole reason subword tokenizers exist, and you will meet them later in this lecture. Arabic root-and-pattern words are the hard case: there is no contiguous substring that means writing shared between kataba, kitāb and kātib, so a tokenizer that only cuts strings cannot see that they are related.

Open a dictionary. walk, walked and walks share one entry; the endings only tell you when the walking happened or who did it. happy and happiness get two entries: an adjective and a noun that names its quality. That single observation is the whole distinction between the two functions an Affix can have. Inflection marks grammar (tense, number, case, agreement) and usually keeps the same Lexeme (the slide's hedge, consistent with SLP3's continuum). Derivation creates a new lexeme, and often changes the part of speech.

SLP3 describes the two ends of the scale. Inflectional morphemes are grammatical morphemes that tend to play a syntactic role, such as marking agreement, the plural -s on nouns or the past tense -ed on verbs; they tend to be productive, often obligatory, and their meanings tend to be predictable. Derivational morphemes are more idiosyncratic in their application and meaning, usually apply only to a specific subclass of words, and result in a word of a different grammatical class than the Root, often with a meaning hard to predict exactly (SLP3, section 2.2). SLP3's own example is care, a noun, plus -ful giving an adjective, plus -ly giving an adverb: two derivations in a row, each moving the word to a new category.

The English slide examples line up with that. walk to walk-ed and walk-s is inflection: every regular verb takes these endings, the meaning is exactly past tense (or past participle) or third person singular, and the word stays a verb. happy to happi-ness is derivation: -ness attaches mainly to adjectives, not every adjective takes it comfortably, and the result is a noun.

Derivation with nothing added

Here is the case that catches people. The noun an email became the verb to email without any affix at all. Nothing was attached, the spelling did not change, and yet a new lexeme with a new part of speech now exists. This is Conversion (zero derivation), also called zero derivation: the creation of a word of a new part of speech from an existing word of a different part of speech without any change in form (Wikipedia, Conversion). It is derivation by the test that matters, a new dictionary entry, even though there is no morpheme to point at.

The same two functions in Arabic

Arabic uses both functions too, and the slide picks an example of each. Inflection: katab-tu كَتَبْتُ I wrote is the Stem katab plus the suffix -tu, which marks first person singular past. The lexeme is still the verb to write; only who and when changed. Wikipedia's overview of Arabic verbs confirms katabtu as I wrote and notes that person, gender, number, tense, mood and voice are specified by changes to the vowels between the consonants along with prefixes or suffixes (Wikipedia, Arabic verbs). Derivation: kitāb كِتَاب book and kātib كَاتِب writer share the root K-T-B but are two lexemes, a noun for the object and an agent noun for the person, made by choosing two different patterns. Do not read a division of labour into that pair. The slide's inflection example happens to use a suffix and its derivation example a pattern, but Arabic uses both mechanisms for both jobs: the broken plural kutub كُتُب books inflects kitāb by changing the pattern alone, and maktab مَكْتَب office derives a place noun with the maCCaC pattern, whose ma- sits in front of the root like a prefix.

Four tests to decide

1. Lexeme: would a dictionary list the two forms under one entry? One entry means inflection.
2. Category: did the part of speech change? A change points to derivation, though derivation can also keep it, as un-happy does.
3. Productivity: does the affix apply to essentially every word of its class with a predictable meaning? Every regular verb takes -ed; not every adjective takes -ness.
4. Position: inflection closes the word. In un-happi-ness-es the derivational pieces sit inside and the plural sits outside. Haspelmath and Sims describe this ordering as a strong cross-linguistic tendency (chapter 5).

Take scissors to walked and you can cut it into walk and ed. Take scissors to sang and there is nowhere to cut: the past tense lives in the vowel, in the middle of the root, and no substring means past. Both words are past-tense Inflection. They differ not in function but in form. That is the second axis of this part, independent of the first: Concatenative morphology attaches segments in a line, and Non-concatenative morphology changes the inside of the word instead.

Concatenative morphology covers everything you have segmented so far: a prefix before, a suffix after, or two roots side by side. cat-s, walk-ed, un-happy and snow-man are all built by placing one contiguous piece next to another. The pieces keep their shape, and you can recover them by cutting. Non-concatenative morphology is defined by what it lacks: word formation and inflection in which the Root is modified and which does not involve stringing morphemes together sequentially (Wikipedia, Nonconcatenative morphology). The slide groups it into internal change, templatic patterns and Suppletion.

Concatenation taken to its limit: Turkish

Turkish shows how far the concatenative strategy can go. The word evlerimden means from my houses, and it is four pieces in a row: ev-ler-im-den. Turkish is a highly agglutinative language, in that much of the grammar is expressed by means of suffixes added to nouns and verbs, and evlerden from the houses is analysed as ev house, -ler plural, -den ablative (Wikipedia, Turkish grammar). Adding the first person possessive -im between them gives the slide's form. Each suffix has a clean boundary and stays recognisable, which is what makes the word so easy to cut (one suffix, -im, still bundles person, number and possession, hence the periods in its gloss).

Three ways to change the inside

Internal change is the English strong-verb pattern. sing, sang, sung (and the noun song) differ only in the vowel of the root. This vowel alternation is called Ablaut, a system of regular vowel variations inherited from Proto-Indo-European (Wikipedia, Indo-European ablaut). The root and the tense are fused in one syllable; there is no -ed to remove.

Templatic patterns are the Arabic case from the first concept, now seen as a mechanism. The root ك ت ب stays fixed in order, and a template of vowels and extra consonants is woven through it: kataba كَتَبَ he wrote, kitāb كِتَاب book, kātib كَاتِب writer. The pieces of the pattern are not contiguous with each other and neither are the pieces of the root, so there is no cut that separates the two morphemes. Watch how the radicals never move while the material between them changes.

Look closely at yaktubu يَكْتُبُ he writes, and you will see both mechanisms at once. The prefix ya- and the suffix -u are concatenative; the Stem shape -ktub-, with no vowel after k and u after t, the pattern CCuC woven through the radicals, is templatic. Arabic words are typically built by wrapping affixes around a root-and-pattern core, which is why an Arabic morphological analyzer has to do two different kinds of work.

Suppletion is the extreme case. The past tense of go is went, and no rule relates the two strings. Suppletion is the use of one word as the inflected form of another word when the two words are not cognates; went was originally the past tense of the verb wend (Wikipedia, Suppletion). Haspelmath and Sims treat it as the extreme case of allomorphy, where the two forms share no phonological material, and the slide lists it under non-concatenative because the outcome is the same: a grammatical change with no segment you can cut off. Suppletion also covers good to better.

Two axes, not one

Function and form are independent. walked and sang are both inflection; one is concatenative and one is not. happiness and kitāb are both Derivation; one is concatenative and one is templatic. When you analyse a form, answer both questions separately: what does the change do, and how is the change realised?

The form axis is the one that decides what a tokenizer can see. Subword tokenization methods such as Byte-pair encoding (BPE) work on contiguous substrings, so they approximate concatenative morphology well: walk can be shared between walked and walking because it is a substring of both. A discontinuous root like K-T-B is never a substring of anything, so the same tokenizer cannot share the writing meaning across kataba, kitāb and kātib. That is SLP3's cross-lingual difficulty made concrete, and it is the hook for most Arabic tokenization research.