They were the films your parents wouldn't allow you to see: "Mark of the Devil." "Flesh Gordon." "Cannibal Holocaust."

They were the films your own parents had kept them from: "The Mermaids of Tiburon." "Blood Feast." "And God Created Woman."

And, yes, they were the films even their parents were once warned off: “Ecstasy.” “Reefer Madness.” “Freaks.”

As long as there have been movie cameras, directors have been focusing them on things polite society would prefer to look away from — gory violence, nudity, drug use, sex.

And audiences have, predictably, been paying money to stare — because nothing is more alluring to fans, particularly adolescent ones, than the forbidden.

This shadow cinema was tagged “exploitation,” and the rundown theaters that showed it, often ’round the clock, were dubbed “grindhouses.” Co-opted long ago by corporations, it’s a vanished tradition now.

But one that’s increasingly, fondly remembered.

Quentin Tarantino and Robert Rodriguez saluted the entire exploitation world in 2007 with their own double-feature, "Grindhouse," including fake trailers (one of which became a real neo-exploitation picture last year, "Machete").

In 2009, "Black Dynamite" similarly re-created the sub-genre of blaxploitation, complete with funky bass lines and ceiling-scraping Afros. The great website trailersfromhell.com continues to curate and comment on those B-movies' boisterous ballyhoo.

And a new documentary, “American Grindhouse,” crams the entire history into 82 minutes — starting off in West Orange, where busy inventor Thomas Edison dreamed up exploitation, too. (His 1894 film of a Spanish dancer was booted out of Asbury Park for indecency.)

Exploitation movies are as old as the movies themselves. In fact, they are the movies.

Warning! This Film Contains Explicit Sex

Just as you can’t have shadows without light, you can’t have “forbidden” films without censorship.

During the early 20th century, American movies existed without any codified system of taboos. Sensationalist exposes such as “A Traffic in Souls” or racist melodramas such as “The Birth of a Nation” made millions by taking on hot-button issues; by the early ’30s, studio films regularly featured barely clad chorus girls and hints of sex and violence.

Eventually the studios pushed too far and public outrage — led largely by the Catholic Church — pushed back. Fearful of federal intervention, Hollywood volunteered to censor itself, drawing up the Production Code. Finally, filmmakers had an actual list of what was forbidden — from nudity and perversion to drug use and sadism.

And a loose assortment of showmen, carnies, eccentrics and charming scoundrels — the “Forty Thieves” — promptly took those prohibitions as their bible.

If this is what the studios couldn’t show, then this is what these fly-by-night showmen would focus on. Smut and dope and sex — they put all of it on-screen as fast and as cheaply as they could, distributing their indie pictures through an adults-only archipelago of run-down theaters, men’s clubs and carnival tents.

For safety’s sake, these shady producers often billed their films as educational, selling worthless “hygiene” pamphlets during intermission and ending every story with an unforgiving moral. It was a canny bit of hypocrisy, but it gave the films a very odd twist — and audiences, no doubt, a slightly depressing experience.

It didn’t matter whether the stories were about the thrills of marijuana (“Reefer Madness”), cocaine (“The Pace that Kills”), betting (“Gambling with Souls”) or general hedonism (“Mad Youth”) — by the final scenes it all came down to addiction, violence and venereal disease, with suicide or prison as the climax.

Tawdry and tedious

Of course, the movies always managed to accomplish their real purpose — providing a few flashes of female nudity, absolutely any way they could. (Drunken skinny-dipping was the most popular excuse, although the most famous of the exploitation pictures, “Mom and Dad,” ended with an explicit childbirth scene.)

A “bold” alternative to Hollywood, these films were simply twisted to a different kink. If studio romances assumed everyone was a virgin before marriage (and, perhaps, even after), exploitation movies insisted only the drunkard and the addict had premarital sex — and afterward the men came down with syphilis and the women killed themselves.

If this was the alternative to sexual repression, perhaps those Andy Hardy films weren’t so bad after all.

It was only after the end of the Depression and the war — as G.I.s came home, Rosie the Riveter was fired from her factory job and gender roles restored — that the moralizing faded. Soon, compliant Playmates greeted readers with smiles on their lips and staples in their bellybuttons; “naturist” films showed the wholesome delights of nude volleyball.

And, slowly, nudity began to show up in feature films as well — in French and Swedish “art” imports, in the special “European versions” of Hollywood potboilers and, finally, in the pneumatic fantasies of Russ Meyer, a world where buxom women ruled.

Relegated to adults-only theaters, these films were winkingly advertised as “for the broad-minded only!” — and, with their corny jokes and above-the-waist nudity, might get only a PG-13 today. But the showmen sold the sizzle and a generation of buttoned-up men in button-downs bought it — along with a still decidedly unliberated take on sexual liberation.

Graphic Violence

If censored studio movies were always playing hide-and-seek with audiences, the greatest hypocrisy came with violence.

Even before the Production Code, graphic violence was verboten. Bela Lugosi’s “Dracula” prowled through two countries without leaving a visible on-screen mark; as many men as Jimmy Cagney and Edward G. Robinson rubbed out, the worst marks the corpses bore was a trickle of blood down their chins.

So, in the early ’30s, exploitation thrillers set themselves apart with a grim ghastliness. In “Maniac,” the titular madman (unconvincingly) puts out a cat’s eye; in “Freaks” (made by MGM, but quickly abandoned to the exploitation circuit) real-life sideshow “attractions” plot revenge against a more monstrous “normal” beauty.

Although sex was always the biggest seller in the grindhouses, gore became a top attraction too. In 1958’s mad “Fiend without a Face,” flying brains attack our heroes (until being bludgeoned into ketchup-doused mush); in 1962’s “The Brain that Wouldn’t Die,” a disembodied head commands a giant freak to tear off a scientist’s arm.

The flow of blood soon became a deluge. Director Herschell Gordon Lewis turned out early ’60s schlockers — “Blood Feast,” “2000 Maniacs,” “Color Me Blood Red” — that depended on close-ups of severed limbs. In 1968, George Romero’s excellent “Night of the Living Dead” featured mindless zombies munching on human intestines.

Then, one month later, the ratings system came in.

at a Theater Near You

Things changed quickly. With studios now allowed to show anything, major corporations took advantage by making their own exploitation films. Major theater chains started booking them. The Forty Thieves — and their grindhouses — lost their niche. How can you sell forbidden pleasures if nothing is forbidden?

It was the bitterest irony of all — they’d finally gotten the freedom they’d demanded, and it was killing them.

A decade before, movies such as “Midnight Cowboy” and “Last Tango in Paris” would have played 42nd Street flea-pits. By the ’70s, they were being nominated for Oscars. The exploiters’ stock themes now drove serious dramas — child prostitution (“Pretty Baby,” “Taxi Driver”), drug addiction (“Lenny,” “Lady Sings the Blues”) even birth defects (“Elephant Man,” “Mask”).

As the era went on, independent filmmakers continued to push the limits of taste, selling their films on shock, sensation and sex. But when Columbia distributed the soft-core “Emmanuelle” in 1974, and Paramount put out the hard-gore “Friday the 13th” in 1980, it was clear exploiters would have to go far, far further than the studios.

Some did, and moved out of theatrical features and onto explicit, straight-to-video pornography. Others didn’t, and moved on to something else. (Lewis, who was always about salesmanship, is now a direct-mail marketing guru.)

By now, the old exploitation business doesn’t exist anymore. It’s been co-opted by corporations.

Its favorite dramatic subjects — religious cults, cross-dressing, teen pregnancy — is the stuff of daytime talk shows. Its most startling scenes — blood-splattered close-ups of abused women — are in “torture porn” movies released, not by one of the back-alley Forty Thieves, but by major multinational studios.

And somehow, that makes it all worse.

Partly because it’s gotten too slick, too convincing. The fact that those early movies were badly made wasn’t their downfall, it was their salvation. We don’t watch “Glen or Glenda” because Edward D. Wood got it on-screen perfectly; we watch because he didn’t. The fact that we’re able to sit through movies such as “The Corpse Grinders” without gagging is because we know it’s all make-believe, a cardboard spook show.

But mostly things have changed because — well, if everything is permitted, where’s the thrill of the taboo? If you want to see silly soft-core sex, you no longer have to sneak into a downtown theater; you just switch on late-night cable. If watching the degradations of idiotic youth is your interest, there’s no need to hunt for a road-show performance of “Reefer Madness”; just tune in to “Jersey Shore.” We’ve all adjusted downward; abnormal is the new normal.

At the time, “Adults only!” nudies and drive-in splatterfests were supposed to be a threat to our nation’s morals.

Now, they’re beginning to feel like some of the last vestiges of its innocence.