http://tvtropes.org/pmwiki/pmwiki.php/UsefulNotes/BeerCommercials

If you've ever watched a television commercial for beer in the United States or watched one on YouTube if you're outside of the States, you might notice something. In the typical American beer commercial, about how many times is someone shown drinking the sponsor's product, compared to, say, someone drinking soda in a soft drink commercial, coffee in one of those commercials, or other beverages? Is it more frequently, or less? If you said less frequently, you'd be correct. In fact, the exact number is zero.

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Television commercials for beer are the only advertisements that never show anyone consuming the product. This is not a legal requirement. It's a holdover from the old Television Code system of the private organization and Television industry political lobbying group, The National Association of Broadcasters. The Television Code was a self-imposed censorship system for making sure stations only ran programs which weren't offensive and followed typical middle-class views, and existed until the late 1970s when stations started to run much edgier fare that wasn't necessarily permitted by the code. Sort of like what happened when comic book publishers decided to drop out of The Comics Code or the way mainstream movie studios stopped following The Hays Code.

The television code had a restriction that an ad for beer (and possibly hard liquor) could not show someone drinking the product. While the code is long since dead, most stations still won't run beer ads where it shows the product being consumed, so as a result, to this day, beer makers do not show their product being consumed in their television commercials.

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Several beer commercials have lampshaded this restriction: