Rhotacism and How It Can Help Your Latin

If you have studied a little Latin, you may have come across an important, yet rather annoying group of nouns. They belong to the third declension, are neuter in gender and end in -us in the nominative singular. They include words like tempus ‘time’, corpus ‘body’ and pectus ‘chest’.

They look like nice second-declension nouns, like masculine dominus and amīcus, but the trouble begins when you look at their various declined forms. Let’s take corpus as the group’s representative.

The twelve forms of corpus ‘body’

With the exception of the nominative, vocative and accusative singular, the -us of corpus ending disappears and -or- appears, seemingly out of nowhere, bringing with it an awful headache for Latin students.

I tell my students to remember whether or not a noun belongs to this group by thinking of an English word that descends from it. For example, tempus, corpus and pectus gave English the adjectives temporal, corporal and pectoral, which reveals the -us/-or- change.

However, we want to do better than tricks like this; we want to understand this phenomenon. This article aims to show you how one linguistic idea, rhotacism, can explain these words.

Neuter? No ending!

The first step is to try to identify what the actual case-number endings of these twelve forms could be — that is, to separate the constant from the inconstant parts of the word, which have nothing to do with its ‘body’ meaning, but rather its function in the sentence. By comparing corpus with other nouns of the same gender and declension, these are the case-number endings that we can identify:

Nominativecorpuscorpor – a
Genitivecorpor – iscorpor – um
Dativecorpor – īcorpor – ibus
Accusativecorpuscorpor – a
Vocativecorpuscorpor – a
Ablativecorpor – ecorpor – ibus
The deconstructed forms of corpus ‘body’

The suffixes we have separated from the rest of the word are exactly the same case-number endings used by other third-declension neuter nouns, such as caput ‘head’ or nōmen ‘name’. Let’s follow that similarity and take a closer look at nōmen.

Nominativenōmen nōmina (nōmin – a)
Genitivenōminis (nōmin – is)nōminum (nōmin – um)
Dativenōminī (nōmin – ī)nōminibus (nōmin – ibus)
Accusativenōmennōmina (nōmin – a)
Vocativenōmennōmina (nōmin – a)
Ablativenōmine (nōmin – e)nōminibus (nōmin – ibus)
The twelve forms of nōmen ‘name’ and their division into stem and suffix.

Nōmen reveals something important. We can see that the case-number suffixes we have identified are simply added to the nominative/accusative/vocative singular form, with only a small change to its second vowel. The N/A/V singular form itself has no suffix; instead, it is both a stem and a complete, stand-alone word.

Being also a third-declension neuter noun, we should apply this analysis to the corpus group of words. When we do, we see that the -us on the end of corpus is not a case-number suffix. It is instead an inseparable part of the word’s stem, to which the real endings can be added. We can therefore call corpus and company ‘s-stems’, and this brings them into line with other neuter nouns of the third and fourth declensions, such as caput or cornū, in that nothing is added to form their nominative/accusative/vocative singular forms.

This idea is corroborated by a sound change that took place in Old Latin: -us on the end of words like both corpus and dominus was originally -os. The older form of corpus therefore was *corpos, which has the same -o- vowel as the one we see in the corpor- forms.

We can therefore deconstruct corpus into the following system of twelve forms:

Nominativecorposcorpos – a
Genitivecorpos – iscorpos – um
Dativecorpos – īcorpos – ibus
Accusativecorposcorpos – a
Vocativecorposcorpos – a
Ablativecorpos – ecorpos – ibus
The underlying structure of corpus

However, one crucial step remains. How do we get from words like *corposa and *corposibus, which we have reconstructed, to their actual documented forms corpora and corporibus? We can do so in a single step: the sound change known as rhotacism.

So, what is rhotacism?

Its name taken from the Greek letter rho, rhotacism is the change of a particular sound into a rhotic sound like [r], the sound known in English as a rolled or trilled R. Many sounds can become [r] over time, but a common candidate is [z] (as in zoo), since both are pronounced with the tip of the tongue and are voiced.

When the case-number suffixes were first added to *corpos, the process of rhotacism began. This is the key to rhotacism. It was through being sandwiched between two vowels that the /s/ in *corposa and *corposibus became voiced [z]. This brought the consonant one step to /r/ and, in time, potentially though other intermediate stages, rhotacism occurred and produced the -or- forms of corpus, tempus and pectus as we know them.

So, in brief:

Intervocalic /s/ became /r/ in Classical Latin

This was a very early change in the history of Latin, but we do in fact have a few examples of Old Latin that display the original /s/ prior to rhotacism. For instance, the text of the Praeneste fibula (seventh century BC) and the Lapis Satricanus (sixth/fifth century BC) attest the names Numasioi and Valesiosio. These would become Numer and Valer by the time of Classical Latin.

Meanwhile, the -os at the end of the N/A/V singular forms of corpos and the rest would become classical -us in an unrelated development, unaffected by any endings or rhotacism.

To sum up, we have the following stages:

StructureStage 1Stage 2Classical result
*corpos + a*corposa*corpozacorpora
*corpos + ibus*corposibus*corpozibuscorporibus
The development of corpus, corpora and corporibus out of their original structure.

Seeing rhotacism in practice

It was not only this group of nouns that rhotacism affected; it had a language-wide effect, and we can find traces of it everywhere.

It explains the English adjectives already mentioned, corporal, pectoral and temporal. These were formed in Latin through the addition of the suffix -ālis. Since -ālis begins with a vowel, it triggered rhotacism on the final consonant of the original noun. Hence *corposālis and *temposālis became corporālis and temporālis.

Let’s look at the nouns ōs ‘mouth’, flōs ‘flower’ and mūs ‘mouse’. They have a different appearance to corpus, and mūs and flōs are also masculine in gender, not neuter. Once again, when the relevant case-number endings were added to these words, in nearly all cases the final /s/ of the stem found itself hemmed in between two vowels and the change into /r/ began. For example, the addition of the genitive singular suffix -is resulted in the genitive forms ōris, flōris and ris.

Nominative/Vocatives > flōs, mūs ēs > flōrēs, mūrēs
Genitiveis > flōris, mūrisum > flōrum, mūrium
Dativeī > flōrī, mūrīibus > flōribus, mūribus
Accusativeem > flōrem, mūremēs > flōrēs, mūrēs
Ablativee > flōre, mūreibus > flōribus, mūribus
The endings and forms of ōs and mūs.

Moreover, the addition of the same adjective ending -ālis involves the same change. This is how the Latin nouns ōs and flōs have given English oral and floral. Latin also derived a verb of the first declension from ōs: ōrāre, meaning ‘to plead, pray’. From this come English orate and adore.

Esse, the Latin verb for ‘to be’, is a tricky verb to learn. It is very irregular. However, it is also important in any language, and doubly so in Latin, because its various forms also provide the endings for some tenses. For example, eram not only means ‘I was’, but it’s also used as a verb ending, as in vīderam ‘I had seen’. It definitely needs learning, and rhotacism can help with this task.

We should first recognise that the present stem of the word is es-. It is from es- that the verb gets its meaning of ‘to be’, and we can see the stem in the infinitive esse and the present tense forms est ‘it is’ and es ‘you are’. Like other verbs in the present tense, es and est are formed with the addition of the simple endings -s and -t (specifically es-s and es-t). Yet in other tenses, the endings that are added to es- start with vowels, such as -am, -ās, -at for the imperfect and –ō, -is, -it for the future. These, having sandwiched the /s/ between two vowels, cause rhotacism. Thus *esam, *esat and *esis become eram, erat, eris, etc.

Present stemEnding and its meaningResult
es- +-s
Second-person singular present indicative
es- +-t
Third-person singular present indicative
es- +āmus
First-person plural
imperfect indicative
es- +unt
Third-person plural
future indicative
es- +-se
Present infinitive

Rhotacism is also responsible for the -re that defines present infinitives. The ‘original’ ending for the infinitive was -se, but it changed when attached to the present stem and its final vowel sound. The rhotacised ending can be seen in the vast majority of infinitives like amāre ‘to love’, monēre ‘to warn’ and venīre ‘to come’, from older *amāse, *monēse and *venīse The original -se has survived in a handful of verbs whose present stems end in consonants, such as esse ‘to be’ and nōsse ‘to know’.

We can also observe rhotacism below the level of whole words. We are taught that there are six genitive plural endings, –ārum, -ōrum, –um, –ium, –uum and -ērum, but we can do better than this. The ‘real’ genitive plural suffix is what unites them, -um, while what remains originates elsewhere. Since the vowel in -um was able to cause the rhotacism of the preceding consonant, we can deconstruct –ārum, -ōrum and -ērum as the results of –ās-um, -ōs-um and -ēs-um. The three affixes that emerge, –ās-, -ōs– and -ēs-, are identical to the accusative plural endings of their respective declensions, and fairly similar to the genitive singular endings –ās and -is.

On this matter at least, I do not want to suggest any ideas for the reasons behind this system (I am aware it likely emerged in pronouns rather than in nouns) or for the precise nature of –ās-, –ōs– and –ēs-; it’s just for another example of the explanatory power of rhotacism.

To conclude

In sum, rhotacism is a sound change that transforms a given consonant in a language into a rhotic consonant like [r]. It has been hugely important for the development of Latin as we know it. Most significant is the change from intervocalic /s/ to /r/, traces of which are still visible in many words, and which is a source of apparent irregularity — yet rhotacism is a very regular change, and recognising its effects is one way to dispel a lot of confusion.


Featured image from here.

7 thoughts on “Rhotacism and How It Can Help Your Latin

    1. That is a superb point – yes indeed, it has been suggested that the ř sound was a short-lived intermediate stage between *z and [r] in Latin, which is a very funny thought for me, prehistoric Romans sounding so Czech! As for the sound itself in Czech, however, that ř emerged from a different origin – it emerged out of the consonant [r] when positioned before the vowels [e] and [i], which are both what we call front vowels, produced with the tongue pushed forward.


  1. Thank you so much for this! I am writing an answer key for an curriculum and I wanted to include a section on rhotacism. Students are always confused by the “blank” that they learn for the m/f nom sing and n nom/acc sing in their charts. Some ask for the “reason” why there is no consistent ending, and why the forms change so drastically. I tried to dive into some more academic papers but the linguistics are far over my head.

    Now I feel like I can boil this down for them and explain that it’s not random changes designed to make them frustrated.

    Thank you!

    Liked by 1 person

    1. Thank you for taking the time to leave this lovely response – I am thrilled that the post is useful for you and your teaching. It’s exactly this that I hoped for when writing it!


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