Why do lawyers still use the words of Roman law?

Because Roman law never really died. Its great law book outlived the empire, was rediscovered in medieval Italy, and trained the lawyers whose codes much of the world still uses. The story starts around 450 BC, when angry plebeians pushed Rome to write its laws down, so judges could no longer apply them as they pleased. According to Livy, the result, the Twelve Tables, was inscribed on bronze and posted in public so every citizen could read it. Centuries later, Cicero said it outweighed the libraries of all the philosophers.

Rome kept building on that foundation for nearly a thousand years. Its final, most ambitious version came from the Eastern Roman emperor Justinian, who from 528 AD set an official named Tribonian and his team to compile and organise centuries of Roman legal writing into one enormous reference work: the Corpus Juris Civilis. Its core volume, the Digest, ran to 50 books.

Then Rome's western half collapsed, and this legal masterwork nearly vanished along with it. Its main surviving copy is a manuscript from around 600 AD, now called the Littera Florentina. The Digest resurfaced in the Italian town of Amalfi, in 1135 or in 1070, depending on the source. Its rediscovery revived the study of Roman law across Europe. In Bologna, a scholar named Irnerius read passages aloud for students to copy, then explained them in notes called glosses. His students became the first 'glossators', who built the medieval program of Roman law studies. Some sources even credit the rediscovery with inspiring the University of Bologna, founded in 1088.

From there, Roman law spread across the continent, mixed over the centuries with Church law, feudal customs and local practices. Napoleon's 1804 Civil Code for France grew out of this tradition. Today this family, called civil law, is the world's most common legal system, used in about 150 countries, including most of Latin America. Its roots go back to Justinian's books.