What is Rhotacism? The S-to-R Revolution

At its heart, rhotacism is a beautifully simple phonetic process. In linguistics, it describes the change of a consonant sound into an ‘r’ sound. The most famous example, and our focus here, is the specific rhotacism that occurred in early Latin. Sometime around the 5th century BCE, the ‘s’ sound, when it appeared between two vowels (what linguists call an “intervocalic” position), began to change.

Why did this happen? Think about how you make these sounds. The ‘s’ is a voiceless sibilant, a hiss made with air and no vocal cord vibration. The vowels on either side of it, however, are voiced. It’s simply easier for the mouth to continue the voicing from the first vowel through the consonant and into the second vowel. This turns the voiceless ‘s’ into its voiced counterpart, ‘z’. From there, the ‘z’ sound, which in Latin was not a stable phoneme, shifted again to the more common and stable ‘r’ sound. It was a chain reaction of articulatory convenience.

This wasn’t an overnight decree. It was a gradual, organic evolution of speech. For a time, both the old ‘s’ forms and the new ‘r’ forms would have co-existed, perhaps with the ‘s’ sounding archaic or rustic, and the ‘r’ sounding more modern and sophisticated.

Uncovering the Evidence: Linguistic Archaeology

So how do we know this happened? We can see the “fossil record” of this change embedded directly within the grammar and vocabulary of Classical Latin, long after the shift was complete. It’s like finding dinosaur bones in your backyard—the evidence is right there if you know what you’re looking for.

Case File 1: The Telltale Nouns

One of the clearest examples comes from third-declension nouns. Take the Latin word for flower, flos. In the nominative case (the subject form), the ‘s’ is at the end of the word, so it remains an ‘s’. But look what happens when we put it in the genitive case (“of the flower”). The original form was flosis. Here, the ‘s’ is trapped between two vowels: the ‘o’ and the ‘i’.

As rhotacism took hold, flosis became floris.

This pattern is everywhere once you start looking:

• mos (custom) became moris in the genitive (not mosis). This gives us the English word “morality.”
• honos (honor) became honoris in the genitive (not honosis). Eventually, the ‘r’ was so influential it even replaced the ‘s’ in the nominative, giving us the familiar form honor.
• corpus (body) became corporis in the genitive (not corposis).

This shows us a “before” and “after” snapshot within a single word’s declension. The nominative form preserves the old sound, while the other cases show the result of the change.

Case File 2: A Verb’s Split Personality

The verb “to be,” esse, provides another spectacular piece of evidence. Look at the imperfect tense, which describes past continuous action: eram, eras, erat (“I was,” “you were,” “he/she was”). The ‘r’ is clearly present.

Now, consider the future tense: ero, eris, erit (“I will be,” “you will be,” “he/she will be”). Again, we see the ‘r’.

Linguists know that the original stem for this verb was *es-. In a form like *eso (the original root for “I will be”), that ‘s’ is between two vowels. Rhotacism kicked in, and *eso became ero. In contrast, the perfect tense form fui (“I have been”) comes from a completely different root, which is why it has no ‘s’ or ‘r’. The verb “to be” is a living museum of linguistic history, showing different forms fossilized from different stages of the language’s development.

From Papisius to Papirius: A Family’s Sonic Makeover

This wasn’t just an abstract grammatical shift; it changed people’s names and family identities. The Roman statesman and writer Cicero himself commented on this phenomenon. He noted that the ancestors of some of the most prominent Roman families of his day had names that would have sounded quite different a few centuries earlier.

The most famous example is the gens Papiria. Their original family name was Papisius. As the s-to-r shift became the new standard in polite society, the family name updated itself to reflect the modern pronunciation. A Papisius in 500 BCE became a Papirius by 300 BCE.

Other examples include:

• The ancient patrician clan of the Valerii were originally the Valesii.
• The Furii were once known as the Fusii.

Imagine your family name changing over a few generations not because of marriage or clerical error, but because the very sound of it went out of style. It’s a powerful reminder that language is tied directly to social identity.

Pinpointing the Past: How Linguists Date a Sound

Linguists can even date this change with surprising accuracy. The process seems to have started in the 5th century BCE and was largely complete by about 350 BCE. We know this because the earliest Latin inscriptions show the intervocalic ‘s’. Furthermore, the Laws of the Twelve Tables (c. 450 BCE) show a mix of forms, suggesting the change was in progress.

A key figure in standardizing the change was Appius Claudius Caecus (the famous censor who commissioned the Appian Way) around 300 BCE. He is said to have formalized the orthographic convention of writing ‘r’ for this sound, cementing its place in the written language and branding the old ‘s’ as officially archaic.

The Enduring Echo of a Forgotten Sound

So, why does a long-dead sound shift in a long-dead language matter? Because it reveals the fundamental truth about language: it is not a static monolith. It is a living, breathing, evolving entity, shaped by millions of mouths over thousands of years in a constant pursuit of efficiency and clarity.

The story of rhotacism is a perfect miniature of this grand process. It shows us how linguists can act as detectives, using clues hidden in plain sight to reconstruct the audible past. It reminds us that the words we inherit, even in English through our vast borrowings from Latin, are not just words. They are artifacts, carrying the echoes of forgotten pronunciations, social shifts, and the quiet, persistent changes that define human history. The next time you see the word “honor” or “moral,” remember the hiss that came before the roll—the sound that, in its own small way, helped shape the voice of Rome.