I was listening to a back episode of WNYC/NPR’s Radiolab the other day and, as I often do when listening to the show, I learned something new. This time, it was the difference between a penitentiary and a prison.

It turns out, the difference between them is like the difference between squares and rectangles ­— all penitentiaries are prisons, but not all prisons are penitentiaries.

Prisons are big, barbed wire-encrusted depositories for criminals, and so are penitentiaries. But what the pens have that your average county jailhouse doesn’t is a system for the rehabilitation of its inmates.

To us, it seems like an obvious idea that wardens would encourage their inmates to evaluate their life choices, but as it turns out, it’s a relatively new practice in the Anglo-American history of bad guy punishment.

Back in the 1700s in England, there were no prisons as we would think of them today. The attitude towards criminal punishment was this: law breakers have proven themselves incapable of abiding by society’s rules, so we therefore need to remove them from society. This was most often accomplished by simply executing the offenders. However, if a criminal hadn’t committed one of the many crimes punishable by death, then they could look forward to another means of removal from society: banishment.

Petty criminals like thieves and debtors were simply shipped to one of the British Empire’s numerous colonies where the law-abiding population wouldn’t have to deal with them.

This penal system is how Australian Aborigines came to have non-marsupial neighbors. But before the Brits were sailing their crooks half way around the world, they were bringing them a much shorter distance, directly across the Atlantic to a scrappy bunch of colonies on the eastern shores of the world’s newest continent. Only three decades after John Smith and his pilgrim pals stepped onto Plymouth Rock, the English were shipping their blights on society to the new American colonies in bunches of 100 at a time.

However, not everyone in England thought the unsympathetic killing or banishing of lawbreakers was such a Christian thing to do. That’s why religiously-minded reformers advocated for change. They thought there was a better way than to continue holding prisoners in dingy, dank, dungeon-like holding cells prior to their criminal sentencing. And the reformers also envisioned a socially productive alternative to execution or banishment. They argued for redesigned jailhouses where both the buildings’ architecture and operation would encourage inmates to reflect on their wayward lives.

So around 1776 when England found its American colonies unwilling to accept any more shipments of prisoners, or tea for that matter, necessity forced the Royal Empire to entertain some of these new ideas on prisoner treatment.

In 1779, the British Parliament passed the Penitentiary Act. The goal was to create a national network of state-owned prisons right there in England. And they wouldn’t just be big jails; they would be penitentiaries: buildings designed to encourage their inmates to become penitent for the crimes they had committed.

They were to be spiritual rehabilitation centers, where inmates would be kept in solitary confinement for long periods of the day—perfect for soul-searching—and would also provide inmates with healthy doses of arduous physical labor—great for exhausting the will to sin right out of them!

Yet, the reformers’ successes were short-lived. Following the passing of the Penitentiary Act, only two penitentiaries were built. And when the Revolutionary War ended, England went right back to its practice of execution and prisoner “transportation” to its various colonies, including Australia.

Yet, the idea of jails being centers for religious rehab endured, and eventually the penitentiary craze hit the young American nation. The Quakers picked up the torch and helped lobby the Pennsylvania state legislature to build the Eastern State Penitentiary, the nation’s first foray into criminal rehabilitation. The ESP would combine extreme isolation with hard labor in hopes of inducing religious epiphany.

Instead, the prolonged lack of human contact combined with the copious amounts of blister-inducing drudgery ended up just making most of the inmates go insane.

But over time, the penitentiary model was tweaked, caught on, and has now become the backbone of the modern penal system.

An ironic endnote: The former favorite dumping ground for British criminals, the American colonies, are now home to the world’s largest incarcerated population, both per-capita and in gross total.