Historical Linguistics: Sound Laws and the Family Trees They Draw

Latin pater, Spanish padre, Sanskrit pitā, English father. Latin pēs, Greek pous, English foot. Latin piscis, English fish. Latin plēnus, English full. Where Latin and Greek have a p, English has an f, and it does so again and again. A coincidence would not repeat itself that neatly.

It is not a coincidence, and the discovery that it is not is the founding moment of historical linguistics. In 1786 Sir William Jones, a judge in Calcutta, told the Asiatic Society that Sanskrit, Greek and Latin were so alike in roots and grammar that they must have "sprung from some common source". But resemblance was the easy part. The hard part, which took the nineteenth century, was showing that the resemblances follow rules. Once they did, you could do something astonishing: write down a word that nobody has heard for five or six thousand years, and be able to say which of its sounds were which.

This piece covers its two great tools, sound laws and the comparative method, and then four big stories they have been used to tell. Every change here is ordinary speech doing what speech does; no language is ranked.

Sound change is regular: Grimm's law as a machine

Sounds change because people speaking quickly to each other shift their habits by tiny amounts. The surprise is that the shifts are not word by word. When a sound changes in a community, it changes everywhere it occurs in the same surroundings, and so a language's whole vocabulary is rewritten by the same rule at once. The Danish scholar Rasmus Rask noticed the pattern in 1818 and Jacob Grimm set it out in the second edition of his Deutsche Grammatik in 1822. We call the Germanic part Grimm's law: the old voiceless stops p, t, k became the fricatives f, th, h, and the old voiced stops b, d, g became p, t, k.

In the 1870s a group of young Leipzig scholars, the Neogrammarians, turned the pattern into a working rule: sound change has no exceptions, and an apparent exception means you have not yet found the reason. They were quickly rewarded. Grimm's law seemed to fail for father (Gothic fadar, not *fathar), until Karl Verner showed in 1875 that the pattern depended on where the stress fell in the original word. Sanskrit pitā́ has the stress after the t; that is exactly where Germanic got a voiced d (English th in father is a later development of the same sound). An exception had turned into a second rule.

Try it. The tool below applies the rules to nine Latin words (Latin kept the old consonants, so it stands in for the ancestor) and compares the result to the first sound of the English cognate. Then break it by turning the rules the wrong way round.

Without JavaScript: with both rules applied in the historical order, all nine words begin with the expected English sound (pater → father, tres → three, duo → two). With the order reversed, duo would become "thuo" because the new t would be shifted a second time.

Applying "b d g → p t k" before "p t k → f th h" would shift one sound twice (d → t → th), and two would come out as "thuo". Linguists call the right order of events relative chronology, and you can often establish it with no calendar dates at all. Sound laws apply to one language, in one period, in one set of surroundings. A few changes seem to spread word by word (lexical diffusion, William Wang and later William Labov). Regularity is a very strong working assumption, not a law of nature.

The method has a good prediction record. In 1879 Ferdinand de Saussure, from comparing vowels across Indo-European languages, proposed that the ancestor must have had some lost consonant-like sounds. No known language had them. When Hittite was deciphered in the 1910s, Jerzy Kuryłowicz showed in 1927 that Hittite's ḫ appeared in exactly the places Saussure's gaps had been. Nearly half a century before the evidence, the rule had predicted a sound.

The Great Vowel Shift: why English spelling lies

English spelling is not random. It is a photograph of how the language sounded around 1400, when printing and standard spelling were forming, and the sounds have kept moving ever since. Between roughly 1400 and 1700 the seven long vowels of English rotated, in what scholars call the Great Vowel Shift. In Chaucer's day bite sounded close to modern "beet", house like "hoose", name like "nah-muh", and goose like "gohss". Read English aloud with Chaucer's vowels and the spelling suddenly makes sense. The letters stayed; the sounds slid.

The usual picture is a chain shift. The two high vowels (ī and ū) could not rise any further, so they broke into diphthongs, and the vowels below them rose to fill the space. Whether the high vowels pushed the others or the others pulled, and whether it really was one "Shift" rather than several linked changes, is argued. Dates are approximate and differ by region. Step through the stages below.

Without JavaScript: bite went from "beet" to "bite", house from "hoose" to "house", meet rose from "mayt" to "meet", meat rose in two steps and merged with meet by about 1700, name rose from "nah" to "nay", boat from "baw-t" to "boat", and goose from "gohss" to "goose".

Regularity again, with a catch. A few words took a different path: great, break and steak kept the older sound and rhyme with late instead of meat. That is not a violation of the rule; it is a reminder that real change spreads through a community, competes with other forms and can be stopped by a social preference. If you meet a "weird" spelling, you are usually looking at a sound that moved after the spelling was fixed.

The comparative method: reconstruction from a cognate table

Cognates are words in different languages that descend from one word in a shared ancestor: English father and Latin pater; Spanish noche and French nuit. They are not the same as words that merely look alike (Spanish mucho and English much have nothing to do with each other), and not the same as borrowings. The comparative method works in three steps. First, line up cognates and find the sound correspondences, sets of sounds that always match. Second, for each set, work out which single ancestral sound could have turned into all of them by natural, common changes. Third, check that the whole system of changes hangs together.

The step that surprises newcomers is the second. You do not take a vote. The Latin cluster -ct- survives in five modern Romance languages in five different shapes, and the shape that wins the vote is not the ancestral one. Try it. This is a cheat in one respect: Latin is written down, so we can check the answer afterwards. In most of the world's families you have no ancestor on paper and the method is all you have.

Without JavaScript: the correspondence is Spanish "ch", Italian "tt", French and Portuguese "it", Romanian "pt". Only an ancestral "kt" reaches all of them by common changes (kt → tt; kt → it → "ch"; kt → pt). The majority vote ("it", two languages) is wrong. Latin has noctem, octo, lactem, factum.

The method has limits. It cannot see back past roughly six to ten thousand years, when too little of the original vocabulary remains to show a pattern. Claims of older kinship such as "Nostratic" are not accepted by most historical linguists, and vocabulary-percentage dating ("glottochronology") is widely distrusted. Modern work uses Bayesian trees, which come next.

Proto-Indo-European and the homeland debate

Applied to Indo-European, the comparative method reconstructs a language with a thousand-word dictionary and a grammar: *ph₂tḗr "father", *dwóh₁ "two", *kʷékʷlos "wheel". The asterisk means "reconstructed, never written". The family has at least ten branches: Anatolian (Hittite; extinct), Indo-Iranian, Greek, Armenian, Albanian, Italic (with Latin), Celtic, Germanic, Balto-Slavic and Tocharian (extinct, once spoken in the Tarim Basin in western China). Nearly half the world's people speak a first language from it.

Where did its speakers live? Two answers dominate, and genetic evidence has changed the argument since 2015.

Hypothesis	Where, when	Spread by	Main supporters
Steppe	Pontic–Caspian grassland, about 4000–3000 BCE (the Yamnaya culture, c. 3300–2600 BCE, is its late form)	Pastoralists with wagons, later horses; migration	Gimbutas; David Anthony; many historical linguists
Anatolian	Anatolia, about 7000–6500 BCE	The spread of farming	Colin Renfrew; Bouckaert et al. 2012

Each side has real evidence. The steppe case rests on vocabulary: Proto-Indo-European seems to have had words for wheel, axle, yoke, wagon and horse, and wheeled vehicles are not found before roughly 3500 BCE. If the language had split by 7000 BCE, those words would have to be later borrowings or older words that changed meaning. The Anatolian case rests on a simple, elegant mechanism (farming expands, languages travel with it), and on computer-built family trees: Bouckaert and colleagues (2012) put the root at about 8,000 to 9,500 years ago. Linguists have criticised the vocabulary data and methods in those trees.

Ancient DNA then changed the picture. Haak et al. and Allentoft et al., both in 2015, found that around 4,500 years ago people with Yamnaya ancestry moved west into Europe on a very large scale, and that central European Corded Ware people were closely related to them. In Britain, Olalde et al. (2018) found that the arrival of the Beaker people from about 2400 BCE was followed by an enormous turnover in ancestry. But DNA is not language: Damgaard et al. (2018) found no steppe ancestry in the Anatolian-speaking Hittite-era people they sampled, which means an Indo-European language arrived in Anatolia without a large migration. Heggarty et al. (2023) built a new tree with ancient languages included and proposed a hybrid: a root around 8,000 years ago, south of the Caucasus, with the steppe as a later centre of spread. In 2025 Lazaridis and colleagues argued from ancient DNA that the earliest ancestors lived between the Caucasus and the lower Volga, around 4400–4000 BCE. Most specialists now treat the steppe region, directly or indirectly, as the source of the main later spread; the Anatolian farmer model survives mostly in a modified form. Treat the exact picture as open.

Without JavaScript: the steppe proposal is about 4000–3000 BCE, the Lazaridis et al. proposal 4400–4000 BCE, Heggarty et al. about 6000 BCE, Bouckaert et al. about 7500–6000 BCE, and the Anatolian farmer proposal about 8000–6500 BCE. A date before about 3500 BCE means the shared wheel words must be loans or meaning shifts.

A related question is how Indo-European reached South Asia. Narasimhan et al. (2019) found steppe-related ancestry appearing there in the second millennium BCE, consistent with the mainstream linguistic view that Indo-Aryan speakers arrived from the northwest. The view that Indo-European originated in India ("Out of India") is held by some scholars and writers but not by most historical linguists, who point to borrowed Dravidian and Munda features in Vedic Sanskrit and to the shape of the family tree. In India the question is tied to national identity, a reason to keep evidence and politics in separate boxes.

Bantu and Austronesian: two great expansions

The same tools work outside Europe, and two of the largest language spreads in human history are best read through them. Bantu is a branch of the Niger–Congo family. Its homeland is in the border region of Nigeria and Cameroon, and its speakers spread over several thousand years across central, eastern and southern Africa. Today several hundred languages belong to it (the count depends on what you call a dialect), including Swahili, Zulu, Shona, Kikuyu and Lingala. Austronesian is the family of Malay and Indonesian, Tagalog, Malagasy, Māori, Hawaiian, Fijian and the languages of Taiwan's indigenous peoples: some estimates give 1,257 languages and about 386 million speakers (Wikipedia, citing Ethnologue-based counts). Taiwan is the most likely homeland, with movement outwards starting about 6,000 years ago.

Expansion	Milestones (approximate)
Bantu	Origin in the Nigeria–Cameroon borderlands; the western stream reaches central Angola about 500 BCE; the eastern stream reaches west of Lake Victoria about 500 BCE; one branch reaches KwaZulu-Natal about 300 CE. The causes and routes are debated.
Austronesian	Taiwan; the Philippines and Indonesia; the Lapita people in the Bismarck Archipelago about 1500 BCE; Fiji, Tonga and Samoa; East Polynesia from about 1025–1120 CE (Society Islands), a wider dispersal about 1190–1290 CE; New Zealand about 1280 CE; Madagascar in the first millennium CE.

What makes these two rewarding for a beginner is that the sound laws are visible in names you may already know. Take the word for "person" in five Bantu languages, all from a root *-ntu with a class prefix: Swahili mtu (plural watu), Zulu umuntu (abantu), Shona munhu (vanhu), Kikuyu mũndũ (andũ) and Lingala moto (bato). And across Polynesia, a regular correspondence links the words for "person" and "sacred":

Gloss	Māori	Tongan	Samoan	Tahitian	Hawaiian
person	tangata	tangata	tagata	ta‘ata	kanaka
sacred, forbidden	tapu	tapu	tapu	tapu	kapu

Samoan writes the sound ng as g. In Hawaiian, Proto-Polynesian *t became k and *ng became n; Tahitian turned *ng into a glottal stop (‘). So Hawaiian kapu is the same word as Māori tapu, and the English word "taboo" comes from the Tongan form. The same method that links pater and father reaches the Pacific. Madagascar adds a twist: Malagasy is an Austronesian language whose closest relatives are spoken in southern Borneo, thousands of kilometres across the Indian Ocean, although the island's people today combine ancestry from both Southeast Asia and Africa.

Language spreading does not prove one population replaced another: in the Bantu case pottery and language do not map one-to-one, and critics call equating them circular. Genetics, archaeology and linguistics each tell part of the story and often disagree.

Creoles: new languages in one generation

Languages also begin. When people who share no language must work together, as on plantations and in ports, they build a pidgin: a small second language with no native speakers. When children grow up with a pidgin as their main language, it can expand into a creole: a full language with its own grammar and a large vocabulary. Haitian Creole (Kreyòl), the first language of most of Haiti's roughly ten million people, takes its vocabulary mostly from French. Tok Pisin, an official language of Papua New Guinea, is spoken by millions, mostly as a second language, with a vocabulary mostly from English. Others include Jamaican Patois, Krio in Sierra Leone, Nigerian Pidgin (tens of millions of speakers), Seselwa in the Seychelles and Papiamento in the Dutch Caribbean. Creoles are not broken versions of their vocabulary sources.

A short sample, glossed in the standard three-line way:

The second line shows something English cannot say in one word. Tok Pisin distinguishes "we including you" (yumi, from "you me") from "we excluding you" (mipela). That distinction is common in the Austronesian languages spoken in Papua New Guinea, a good example of a creole taking its grammar from the language of its speakers rather than from its vocabulary source. Now look at Haitian Creole's verb.

Without JavaScript: in Haitian Creole the verb manje never changes. Li manje "he/she eats"; Li te manje "he/she ate"; Li ap manje "he/she is eating"; Li pral manje "he/she will eat". English has to change the verb itself (eat, eats, ate, eating).

Where does this grammar come from? Derek Bickerton's Roots of Language (1981) proposed that creoles formed by children with no adequate model reveal an inborn "language bioprogram". Critics point instead to the African languages that plantation populations already spoke (Claire Lefebvre argued Haitian Creole owes much to Fon and other Gbe languages) or to ordinary mixing from many inputs (Salikoko Mufwene's "feature pool"). John McWhorter argued creoles have simpler grammars than older languages; Michel DeGraff and others reject that. These questions are live. What is not live is whether creoles are real languages: they have native speakers, literature, courts and radio.

What a graduate just sees

• A resemblance between words proves nothing; a regular correspondence between sounds is evidence.
• You reconstruct the ancestor from the changes that are plausible, not from the form most daughters share.
• English spelling is a snapshot of c. 1400; the Great Vowel Shift is why "bite" and "meet" are spelled the way they are.
• Sound laws have reasons behind their exceptions (Verner's law) and sometimes real exceptions (lexical diffusion).
• Words, genes, pots and peoples spread at different speeds and do not travel as a bundle.
• There is no "pure" language and none that is a corrupted one. Creoles are simply the newest.
• The comparative method stops being useful somewhere beyond 6,000 to 10,000 years, and claims past that need extra caution.

Where it shows up next

• Sociolinguistics (36/26): sound change is not only a story about the past; it is driven by social groups, prestige and identity in the present.
• Languages of the World (36/29) and Phonetics & Phonology (36/21): families, typology and the sound systems that changed.
• Computational Linguistics (36/30): phylogenetic trees for languages use the same Bayesian tools as biology; Heggarty and Bouckaert used them.
• In the world: archaeology and ancient-DNA debates, place-name research, legal and educational language policy, the revival of languages such as Māori and Hawaiian, and the documentation of endangered languages before they are lost.

If you want to go deeper

• 📘 Textbook. Lyle Campbell, Historical Linguistics: An Introduction (MIT Press, several editions), is the standard undergraduate text; Terry Crowley and Claire Bowern, An Introduction to Historical Linguistics (Oxford University Press) is another, with many Pacific and Australian examples.
• 📘 Gentler. David Anthony, The Horse, the Wheel, and Language (2007) makes the steppe case at length; David Reich, Who We Are and How We Got Here (2018) explains the ancient-DNA side; John McWhorter, The Power of Babel (2001) is a lively introduction to language change.
• 🌐 Free text. The open textbook Essentials of Linguistics (Anderson et al., on the eCampusOntario open library) has chapters on language change; Glottolog (glottolog.org) is a free catalogue of families.
• 🌐 Free course. MIT OpenCourseWare 24.900 Introduction to Linguistics (Spring 2022) has sessions on language change and on language endangerment, a good free base for this subject.