Even on your first trip to a skin flick parlor, what first strikes you is not the bare bosom on the screen, but the people in the audience. There are about fifty lean men, sitting bolt-upright and silent, and scattered evenly throughout the theatre. Un-written skin flick code says no one may sit within three seats of anyone else, even on crowded weekends.

The same code makes it faux pas to laugh or smile at even skin flick comedies (which are extremely un-popular and therefore used only as second or third features). Skin flicks are serious business.

In the early evening, it is nor uncommon to find an M.I.T. fraternity or a half dozen Mass General nurses seated near the back of the theatre. Even a college couple "out slumming" can be seen occasionally. Then there are squeals and titters, most often from the M.I.T. section.

BY ten o'clock, women and students clear out, and the theatre settles into total silence. The only sound is the usher, thrashing about as he watches the screen over his shoulder.

Skin flicks themselves are rarely as interesting as skin flick audiences. The general plotline never changes: take as many girls as imagination permits and get them as undressed as law permits. Undress them as quickly as possible, and leave them undressed for as long as possible.

If you've seen one skin flick, you've seen them all. Even the women look the same from flick to flick, though their names change. Good directors manage to work in a wide selection of breasts and nipples, and occasionally several skin colors, since that is the only variety possible.

The number of perversions available to writers is much more limited. Voyeurism makes for a good opener--usually a man spying on women at a swimming pool, private beach or health resort. "The Girl Killer" begins with a sexual sadist and necrophiliac watching women sunbathe on a tenement roof through the breast-shaped lens of binoculars.

NEXT comes the heterosexual scene, with the man and woman clad only in shorts and panties--though not always in that order. After a few minutes of French kissing filmed at point-blank range (a two-story tongue can be terrifying), the woman's garment is removed, though the man's is not. The camera is likely to take more interest in the garment than the girl.

Orgasm is portrayed with a close-up of the woman's twisted face or, less frequently, with an out-of-focus pan through the trees or across the ocean. The latter is not unlike short story accounts submitted by Cliffies to English C sections. Occasionally an oral-genital act is implied, though always with a shot of the passive partner's face.

Sadism comes next, with whipping and strangulation sharing current popularity. The man usually strangles the woman with a nylon stocking or his bare hands until a single drop of blood rolls from the corner of her mouth.

Strangely enough, most skin flicks also have at least one Lesbian scene. The most common form finds two semi-nude women caressing in bed. In an imaginative variation, they shave the posterior of a third female friend. Sometimes one woman begins the encounter disguised as a man. Male homosexuality, however, is carefully avoided.

FINALLY, there is always something for the transvestite. Rare is the skin flick that does not have at least one man in full feminine harness, including bra, belt and garters. Occasionally two or more men go full drag a la jack Lemmon '47 and Tony Curtis is Some Like It Hot.

Despite all these efforts, the best performances still come from the audience. During one recent from the audience. During one recent film in which the screen goes black and a passionate panting can be heard, a colored kid in the first row cut loose with an enviable imitation of a cat in heat that made even the usher laugh.

Coming attractions remain the most popular reel in any skin flick program. Even the management comes down to see them. Unlike the films they advertise, they are fast-paced and contain only the most provocative scenes. The promise of sexual perversion is often strong:

"Helga beats men into submission and then forces them to perfrom un-natural acts with her. You might like Helga. Some men do. I CRAVE YOUR BODY reaches out for new limits in sexual stimulation. See Johnny Mustang stimulate Helen Baker to the height of sexual passion ("Oh, Johnny!"). Perversion in all its gory detail . . .I CRAVE YOUR BODY!"