40 Latin Legal Terms, Explained by an Actual Latin Course

Every other page on this subject gives you a glossary. A glossary is the least useful way to learn legal Latin, because these are not words. They are fragments of Latin sentences that got detached from their sentences several hundred years ago, and the endings they are still wearing are the whole point. Mens rea and actus reus differ by one letter because one noun is feminine and one is masculine. Prima facie is in the ablative. Affidavit is a verb, and has never once been a noun in Latin.

Learn them grouped by grammar and forty terms collapse into about six patterns. Here are all forty, sorted by what they are actually doing.

1. Verbs that got jobs as nouns

The oldest trick in legal Latin: take the first or most memorable word of a writ, and let it name the document.

Caveat is the one worth stopping on, because it explains a construction most learners meet late. Latin has no imperative for let him do something: an imperative can only address the person in front of you. So it uses the subjunctive in a main clause, and that is a jussive subjunctive. Caveat emptor is let the buyer beware, with emptor in the nominative because he is the subject of the verb. English then took caveat, a third person singular verb, and made it a noun you can pluralise, so contracts now have caveats.

Habeas corpus works the same way one person over: a second person subjunctive giving a formal instruction to the gaoler. Certiorari is a passive infinitive and means to be more fully informed, which is exactly what a superior court is asking for when it calls up the record of a lower one. Mandamus is a plain first person plural: we command, in the royal voice, which is why it survives in jurisdictions that kept the prerogative writs.

2. Prepositions taking the ablative

Ten terms, one rule. De, ex, pro, sub and in (when it means position rather than motion) all put their noun in the ablative.

Pro bono is the one people truncate without noticing. The full phrase is pro bono publico, for the public good, and bono is a neuter adjective standing in for the noun. Nobody says the third word any more, which is fine, but it means the phrase's grammar looks headless.

In camera is a favourite of headline writers who imagine a photographer. Camera is simply a vaulted room, later a judge's private chamber, and the phrase means the hearing was held in the judge's room rather than in open court. The optical instrument got its name from the same word by way of the camera obscura, the dark room.

In flagrante delicto deserves better than the use it gets. It is an ablative absolute in miniature: while the crime is burning, that is, mid-offence. It applies perfectly well to a burglary. Popular usage has confined it to one specific sort of crime scene, which says more about popular usage than about the Latin.

3. Prepositions taking the accusative

The other half of the rule. Ad, inter, per, ultra and versus all take the accusative.

Inter alia is neuter plural: among other things. If you mean among other people, the correct form is inter alios, and if all of them are women, inter alias. Barristers get this right and journalists do not.

Ad litem and ad hoc are the same construction with different nouns, so a guardian appointed ad litem is appointed for the purposes of this suit only, and dissolves when the suit does. Ultra vires is an act beyond the powers an authority actually has, and its opposite, intra vires, is almost never said aloud, because nobody litigates about things that were allowed.

And it is per se, never per say. Se is a pronoun. There is no verb in it.

4. Bare ablatives, no preposition attached

Latin can put a noun in the ablative and let the case do the work of a preposition. Five of these are everywhere.

Mutatis mutandis is the most impressive thing in the whole of legal Latin and gets used as though it meant and so on. It is an ablative absolute built from two forms of the same verb: mutatis, a perfect passive participle, having been changed, and mutandis, a gerundive, the things that need changing. The clause means: once you have changed everything that needs changing, the rest applies. That is a genuinely precise instruction, and it fits in two words.

Bona fide is an ablative meaning in good faith, and English reanalysed it as an adjective, so we now write a bona fide offer, which in Latin is not a possible phrase. The nominative bona fides then arrived separately and got treated as an English plural: his bona fides are impeccable. That is a Latin feminine singular taking a plural verb. There is more of this in Latin phrases used in English.

5. A noun and an adjective, agreeing

Four terms, one rule: the adjective matches its noun in case, number and gender. Change the noun and you must change the adjective.

This pair is the best advertisement for Latin adjective agreement in existence. Mens rea and actus reus are the same adjective twice. Nothing about guilt changes between them; only the gender of the noun does. Once you see that, the -a and -us stop looking like arbitrary legal decoration.

The plural of obiter dictum is obiter dicta, and of res judicata, res judicatae. Judges say dicta constantly and half the profession thinks it is a singular.

6. Genitives hanging off a noun

A genitive means of, and Latin usually puts it after the noun it depends on.

Three of these five use a gerund, the verbal noun that gives English -ing: proving, deciding, standing. Latin declines it like a noun, so probandi is of proving. That is one construction covering the burden of proof, the binding part of a judgment, and the right to bring a case at all. If gerunds are on your list of things to avoid, legal Latin has bad news, and gerunds and gerundives has the fix.

Corpus delicti is not a corpse. It is the body of the offence, meaning the substance of the crime: the fact that a crime happened at all. Crime fiction has ruined this one so thoroughly that the misunderstanding is now more common than the term.

7. Two whole sentences

Res ipsa loquitur is a complete Latin sentence with a subject and a verb, and it is the only one most people meet that uses a deponent: loquitur looks passive and is active. The doctrine says the accident could not have happened without negligence, so the facts argue the case by themselves. Quantum meruit means a claim for the value of work actually done, in the absence of an agreed price.

Three more full sentences worth knowing, without the table: stare decisis, to stand by things decided, an infinitive plus an ablative; nemo dat quod non habet, nobody gives what he does not have; and volenti non fit iniuria, no wrong is done to one who consents, where volenti is a dative present participle.

The misuses worth correcting

• Prima facie does not mean obvious. It means sufficient on first inspection, pending rebuttal. The whole point is that it can be rebutted.
• De facto and de jure are opposites and get swapped constantly. De facto is what is happening; de jure is what the law says.
• Inter alia is neuter. People are inter alios.
• Obiter dicta and res judicatae are plurals. Using dicta as a singular is now so common it is nearly standard, which does not make it right.
• Ad hoc means for this specific purpose, which is a sign of planning. English has drifted it to mean improvised, the opposite.
• Per se, not per say.
• In camera involves a room.

What to take away

• Legal Latin is frozen sentences, not vocabulary. The ending is the meaning, so read the endings first, exactly as in the Latin case system.
• Prepositions fix the case: de, ex, pro, sub and stationary in take the ablative; ad, inter, per, ultra take the accusative.
• Affidavit, mandamus, caveat, fiat and habeas are conjugated verbs. English made nouns of them.
• Mens rea and actus reus are one adjective agreeing with two nouns of different gender. That is the entire difference.
• Genitives hang off the back, and three of the big ones are gerunds: probandi, decidendi, standi.
• Say it the way your jurisdiction says it, and know what it would have sounded like in Rome.

If you want the rest of the machinery rather than forty samples of it, the Latin course builds the cases, the subjunctive and the participles in order from nothing. And if you are here because you like the sound of the phrases, the honest companion piece is Latin tattoo phrases that are actually grammatical, where the stakes of getting an ending wrong are permanent rather than merely embarrassing.