Like much else that’s weird and complicated about American criminal justice, we can blame the English for our jury system. Thanks, guys. Among many other things, we inherited two kinds of juries: petit (pronounced petty) and grand. And no, that doesn’t indicate that one kind is only interested in stupid things while the other wears diamonds and lives in a mansion. The names are French—small and large—because back when they were invented, members of the English nobility were speaking French. Which tells us how old the practice is; it dates all the way back to the Norman Conquest. And the names simply refer to the fact that grand juries are bigger than petit (traditionally, 23 members instead of 12).

Here’s what happened back in 1166. As part of his attempt to unify England, King Henry II created royal courts that rode around, hearing cases in various jurisdictions (shires). These were called circuit courts, a name that carries over into our own federal judiciary today. Circuit courts helped solidify Hank’s rule as well as bringing more uniformity to English law (creating, in fact, a common law). But the process created problems. One of those was that a period of time passed between court sessions in a particular place, and the judge who eventually showed up had no clue what had gone down in his absence.

But Hank had a solution for that too. When the judge appeared, a group of the most prominent local men would appear in front of the sheriff and report—under oath—all the crimes that had occurred in the interim. Jury comes from the French word meaning to swear. So from the beginning, the grand jury’s job was to consider evidence and initiate criminal proceedings.

Jury roles evolved over the centuries, but grand juries made their way into the Magna Carta in 1215 and the English Bill of Rights in 1689, and then hopped the pond and settled into the 5th Amendment to the US Constitution. Settled pretty firmly, in fact, because while the UK has pretty much abolished them and the rest of the world either dumped them or never adopted them to begin with, the US still uses grand juries. (So does Liberia. That’s it.)

How they’re used varies by jurisdiction, as does the size of the grand jury and the manner in which it’s chosen. One thing is universal, however: unlike petit juries, grand juries do not decide guilt.

So what do they do? Thanks to the 5th Amendment, the prosecutor must present evidence before a grand jury in all federal cases. The grand jury then decides whether there’s sufficient evidence to constitute probable cause. If so, it issues an indictment  (yes, more French there) containing the formal charges, and the case proceeds. If not, the initial charges are dropped. The prosecutor can either give up or try again (with the same or different charges; double jeopardy protections don’t apply).

Now, for various complicated reasons, the grand jury clause of the 5th Amendment doesn’t apply in state proceedings. A few states, like New York, require them anyway. Most others keep it as an option. The prosecutor can choose to go before a grand jury or can instead go in front of the judge, a proceeding called a preliminary hearing (or prelim). The latter course of action is much more common, but for strategic or political reasons DAs do occasionally choose the grand jury route.

Grand juries are usually big—23 is a common size. They have the power to order (subpoena) people to appear before them for hearings and, to some extent, the power to conduct investigations.

A few jurisdictions such as California also use civil grand juries. These generally investigate potential government misconduct.

For hundreds of years, people have referred to criminal grand juries as the Sword and the Shield. Shield because they can protect citizens from malicious prosecutions; sword because they have the power to bring justice against wrongdoers. It’s debatable how well they serve either of these functions, especially today. But they’re still interesting creatures and—this is where you come in—serve as wonderful potential plot points.