Imagine holding a fossil in your hand. Not of a dinosaur or an ancient plant, but of a language—a snapshot frozen in time, capturing the exact moment when one tongue was transforming into another. For the French language, that fossil exists. It’s not a poem or a story, but a dry, political document from the 9th century: a military pact known as the Oaths of Strasbourg.

Dated to February 14, 842, this document is widely considered the birth certificate of French. It contains the oldest surviving text written in a language that is recognizably not Latin, but a new Romance vernacular—the direct ancestor of modern French. To understand its significance, we must first step back into the chaotic world of the fractured Carolingian Empire.

A Kingdom Divided

When Charlemagne died in 814, he left behind a vast, unified empire. His son and successor, Louis the Pious, lacked his father’s political skill and struggled to maintain control. Upon Louis’s death, his own three sons—Lothair I, Louis the German, and Charles the Bald—plunged the empire into a bitter civil war, each vying for the largest piece of the inheritance.

The two younger brothers, Louis the German and Charles the Bald, found a common enemy in their ambitious elder brother, Lothair. After a brutally bloody encounter at the Battle of Fontenoy in 841, they decided to formalize their alliance. They met in the city of Strasbourg with their armies drawn up, ready to swear a public oath of mutual support. And it was here that a practical problem led to a linguistic miracle.

Speaking to the Soldiers

An oath is only effective if the people it binds can understand it. The armies of Louis and Charles spoke different languages.

• Louis the German ruled the eastern Frankish lands, and his soldiers spoke a Germanic dialect we now call Old High German.
• Charles the Bald ruled the western Frankish lands, and his soldiers spoke a local, evolving form of Latin known as the lingua romana rustica (the “rustic Roman language”), which we can call Proto-French.

To ensure total clarity and commitment from both sides, the leaders performed a brilliant piece of political theater. Each king swore the oath not in his own language, but in the language of his brother’s troops. Louis the German recited the oath in Proto-French, so Charles’s soldiers could understand him directly. Charles the Bald then recited the same oath in Old High German for Louis’s men. Afterwards, the troops themselves swore an oath of allegiance, each in their own native tongue, promising not to support their king if he broke the pact.

The historian Nithard, a contemporary and cousin to the kings, recorded the entire event, including the precise wording of the oaths. His record is our time capsule.

The “Fossil” in the Text

While the Old High German portion is invaluable to linguists, it’s the Romance text, sworn by Louis the German, that marks the birth of French as a written language. Let’s look at the words he spoke:

Pro deo amur et pro christian poblo et nostro commun salvament, d’ist di en avant, in quant deus savir et podir me dunat, si salvarai eo cist meon fradre Karlo et in aiudha et in cadhuna cosa, si cum om per dreit son fradra salvar dift, in o quid il mi altresi fazet, et ab Ludher nul plaid nunquam prindrai, qui meon vol cist meon fradre Karle in damno sit.

In English, this translates roughly to:

“For the love of God and for the Christian people and our common salvation, from this day forward, as God gives me knowledge and power, I will protect this brother of mine, Charles, with my help and in every thing, as one ought to protect one’s brother, so long as he does the same for me, and I will never knowingly make any pact with Lothair that would harm this brother of mine, Charles”.

To a speaker of Classical Latin, this text would be jarring and “incorrect”. But to a linguist, those “errors” are the whole point. They show us precisely how Vulgar Latin, the spoken language of the Roman Empire, had morphed into something entirely new.

From Latin to Romance: The Evidence

Let’s compare a few words and phrases to see the evolution in action:

• Loss of Case Endings: In Classical Latin, “for the love of God” would be Pro Dei amore. The word Dei is in the genitive case (“of God”). But in the Oaths, it’s pro deo amur. Deo looks more like the dative or ablative, but what’s really happening is that the complex case system is breaking down. The function of the noun is now determined by prepositions (like pro) and word order, not by its ending. This is a defining feature of all Romance languages.
• Vanishing Consonants: The final “m” of Latin accusative nouns, like in fratrem (brother), has disappeared to give us fradre (which would later become the modern French frère). The “t” in fratrem softened to a “d” sound, another common phonetic shift.
• New Vocabulary and Word Order: The word for “thing”, cosa, comes from the Latin causa (a cause, a legal case), which had replaced the classical word res in common speech. The word order is also moving away from the flexible Latin structure towards the more rigid Subject-Verb-Object pattern we see in modern French.
• A New Future Tense: The text has salvarai eo (“I will save”). This construction—the infinitive verb (salvare) plus the verb “to have” (habeo, which became ai)—is a Romance innovation. It literally meant “I have to save”, and this formula became the standard future tense in French (je sauverai) and other Romance languages.

A Monument of History and Language

The Oaths of Strasbourg were a political success. The alliance held, Lothair was defeated, and the following year, the Treaty of Verdun (843) officially partitioned the Carolingian Empire into three kingdoms. These new territories laid the groundwork for the modern nations of France (Charles’s West Francia), Germany (Louis’s East Francia), and a middle kingdom, Lotharingia, which was fought over for centuries.

The Oaths are therefore monumental on two fronts. Historically, they mark the formal division of Charlemagne’s Europe. Linguistically, they provide the first undeniable proof of a new Romance language, distinct from its Latin parent, being used in a formal, official capacity.

It is a rare and precious thing to witness the birth of a language. The Oaths of Strasbourg allow us to do just that, offering a window into a world where Latin was fading from common speech and the voices of new nations were, for the first time, being put to parchment.