A window into Hell.
The Alterpiece Tryptich was an old idea by the time that Heironymus Bosch got ahold of it. Three painted panels in an elaborate frame, often telling a story from right to left. Giotto's Stefaneschi Triptych for St. Peter's cathedral is a good example of what a tryptich should look like — Christ, serene, surrounded by adoring disciples. But Bosch had a different story to tell.
The Garden of Earthly Delights is a folding tryptich, the left and right panels fold inwards, and when closed is shows a meditative image of the world during its creation, painted in the gray-green grisaille common to Netherlandish triptychs of the time. A moment of solemnity before the leaping madness inside is revealed.
We really don't have to introduce The Garden of Earthly Delights. It's one of the most famous, or infamous artworks in history, and for good reason. Opening the tryptich reveales three panels, on the left, the Garden of Eden, in the center, the titular Garden of Delight, and on the left, Hell. Each panel vibrates with activity — dozens of tiny figures indulge in every possible vice, perversion and torture. It's remarkable. It's a tour-de-force. It's an incredibly imaginative depection of sin and it's extrordinary punishments. Take a look, zoom in, explore.
The Garden of Earthly Delights was so successful in it's time that it spawed imitations by numerious artists. Even today, it's influence can be felt in the work of artists like H.R. Geiger. Bosch worked in a time of extreme religious piety, and he found a loop hole. He created a caution against depravity, that nonetheless gave his viewers a twisted peek into their darkest fantasies.