What Is a Sound Change? Why Languages Drift Apart

The central discovery of nineteenth-century linguistics was that sound changes apply to sounds rather than to words, so a shift affecting a particular sound in a particular environment affects every word containing it. That regularity is what makes historical reconstruction possible, since exceptions demand explanation rather than being shrugged off, and the exceptions turn out to have identifiable causes including borrowing from another language or dialect after the change had run, analogy with related forms, and the change being conditioned by an environment that was not initially noticed. The principle, formulated by a group of German scholars in the 1870s, was contested at the time and became the foundation of the comparative method. Its practical consequence is that comparing languages systematically reveals correspondences, so a sound in one language regularly corresponding to a different sound in another indicates descent from a common ancestor rather than coincidence, and the ancestral form can be reconstructed from the correspondences.

Several types recur across unrelated languages, which suggests they reflect how speech is produced and perceived:

• •Assimilation, where a sound becomes more like a neighbour, which is why the prefix in-turns into im- before a lip consonant
• •Lenition, where sounds weaken between vowels, so a stop becomes a fricative and then may disappear, which is how Latin words lost consonants on the way to Spanish
• •Deletion, including the loss of sounds at the ends of words, which has reshaped the grammar of several languages by removing the endings that carried it
• •Epenthesis, where a sound is inserted to ease a difficult sequence
• •Vowel shifts, where the whole system of vowels rotates, of which the Great Vowel Shift in English between roughly 1400 and 1700 is the standard example and the reason English spelling matches its pronunciation so badly
• •Metathesis, where sounds swap order, which produced the modern form of several ordinary words

The observation that European and Indian languages are related was made in the late eighteenth century, most famously in a lecture by William Jones in 1786 noting resemblances between Sanskrit, Greek and Latin too systematic to be accidental. Working out the relationships required identifying regular correspondences, and Grimm's law, formulated in the 1820s, described a systematic shift affecting consonants in the Germanic branch, explaining why an English word beginning with f corresponds to a Latin word beginning with p, as in father and pater or foot and pes. Apparent exceptions to that law troubled the field until Karl Verner explained them in 1875 by identifying a conditioning factor involving where the stress fell in the ancestral language, which converted the exceptions into a further regular rule and strengthened the whole approach enormously. That episode is a standard illustration of how an anomaly properly investigated becomes evidence rather than an embarrassment.

Sound change is not confined to the past and is observable in progress. Sociolinguistic work from the 1960s onward, particularly by William Labov, established that variation in a community correlates with age, class, sex and the formality of the situation, and that comparing age groups reveals changes moving through a population, which allows a change to be studied while it is happening rather than reconstructed afterwards. Documented ongoing shifts include vowel changes across regions of North America and Britain, the spread of particular consonant pronunciations through urban populations, and changes in intonation patterns. Media exposure alone appears to be a weak driver, since change spreads chiefly through face-to-face contact within social networks, which is why accents persist despite universal broadcasting. Complaints about declining standards accompany every observed change and are as old as records of language, which is itself a reliable regularity.

Changes apply to sounds in environments rather than to individual words, so every word containing the sound is affected, and the apparent exceptions turn out to be borrowings or further conditioned rules. That regularity is what makes languages reconstructable. Grimm's law explains why English father matches Latin pater, and Verner resolved its exceptions in 1875 by finding where the ancestral stress fell.