In 1968, the Motion Picture Association of America replaced their nuance-free, prohibitive Production Code (a.k.a. the Hays Code, the reason couples in old movies had to sleep in separate beds) with a system of alpha-numerical ratings denoting mature material or its absence. In 1990, they introduced the rating of NC-17 for films with such explicit content that they’d be unsuitable for any children under seventeen, regardless of adult accompaniment. In the sweet spot between those two points, the X reigned supreme.

The MPAA reserved their distinction of ‘X’ for only the most salty-mouthed, violent, or sexually libertine content, making it an instant marker of controversy when deployed for a new release. Many pornographic films took advantage of the MPAA’s failure to copyright their ratings system and branded themselves with an X to affect the appearance of extra-salaciousness, but some more well-regarded films closer to the mainstream earned the dubious honor as well. Below, we’ve compiled a rundown of these X-rated classics available to stream now — just make sure nobody’s watching over your shoulder.

Gallery — The Best NC-17 Rated Movies in History:

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