This week I’m in the middle of my fifth trip in two months, and this time I’ve transported across the country to Philadelphia. If I’m lucky I’ll get a chance to tour Eastern State Penitentiary (ESP). If I make the tour, I’ll post pics.

So today’s a good chance to discuss two competing early models of prisons. The first of these, called the solitary or Pennsylvania system, was pioneered at ESP in 1829. As the name suggests, inmates were locked up alone and were visited only by prison staff and clergy. The inmates were supposed to work during the day. It was thought that this system was more humane than the models of incarceration otherwise in use, in which large groups of prisoners shared cells–often leading to violence, victimization, and further criminal behavior. It was also assumed that solitary confinement would give inmates the opportunity to consider the errors of their ways and reform themselves. That’s why these prisons were called penitentiaries.

The competing model was called the silent or Auburn system, after Auburn prison in New York. In these prisons, inmates spent the night in solitary cells. During the day they worked together, but they were not allowed to speak to one another. Corporal punishment (flogging) was widely used. This treatment was intended to dehumanize them and turn inmates into obedient factory workers.

The Auburn system was ultimately more popular. For one thing, inmates kept in solitary confinement for extended periods tended to go crazy. Prison overcrowding—a problem even in the 19th century—soon made single-occupancy cells impossible. And from a practical viewpoint, inmates could achieve more work and more kinds of work when they were in groups. This factor became important when people realized the potential profits from prison labor.

Both the Pennsylvania and Auburn systems eventually fell out of favor, replaced by other models of imprisonment. But if you have a story with a 19th-century prison setting, you might want to research which of these models was in use in your jurisdiction.

You’ve heard it mentioned in countless prison movies—the hole, prison slang for solitary confinement—and it turns out, the truth is pretty ugly.

Let’s define the term first. An inmate in solitary confinement will be housed alone. Generally, the only humans he interacts with will be prion guards, and even that will be brief. He will remain in his cell almost all the time, perhaps being permitted out for a brief period to shower or exercise (also by himself). Solitary has other nicknames too, such as adseg (short for administrative segregation). Solitary is often placed in an area called the SHU, for Secure Housing Unit.

Some of the first US prisons used solitary for all inmates. The theory was that prisoners kept alone would have time to think about their erroneous ways and reform themselves. What happened instead was that, deprived of human contact, the prisoners went crazy. Because solitary confinement was also expensive and restricted inmates from doing most kinds of work, it was soon replaced by other prison models.

But solitary remains in use for several purposes: 1. Punishing prisoners for crimes or misconduct committed while they are locked up; 2. protecting vulnerable prisoners from other inmates; 3. restricting suicidal inmates’ access to objects they could use to harm themselves; and 4. segregating inmates determined to be especially risky, such as gang members.

Conditions in solitary vary a great deal. In some cases, they are very poor. The cell may be extremely small and sometimes devoid of everything except some kind of toilet. The inmate may even be naked. Prisoners in solitary are often given little to do with their time.

There are no limits on the length of time someone can spend in solitary. Some have spent decades there. Many advocates argue that this amounts to torture, especially when the prisoners are young or mentally fragile. It’s also poor policy, in that most prisoners in solitary will eventually be freed but will have a sky-high risk of recidivating.

The US uses solitary to a great extent, and some people claim that this amounts to human rights violations, especially when the confinement is long-term.