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Can you tell us the series in question?

Even if a manga is rated +18 it doesn't mean it's hentai, not to mention that the "hentai" label is used for mangas that have explicit sex and focus on this. For example "Ikitousen" (a manga that i hate, so i don't know how you spell it) is ´+18, border line hentai and not "hentai" even tough there is sex on it. It might be the case in your manga.

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The question now here is if hentai is a demographic or a genre. MU lists it as a genre, and I agree.

My take on this? What I consider "hentai" would be those that are sexually explicit to the point that censorship is necessary. I have read/am currently reading Vitaman titles such as Velvet Kiss. Virgin na Kankei, and Glass no Megami and I know that those are not censored because of the mere fact that they aren't sexually explicit enough to warrant it (genitalia isn't even shown). And given their content I agree with labeling them as "seinen."

Perhaps the best way to answer this is for an admin to tell us how MU defines "hentai."

Quote from JakeeDylan

Today I was looking at Vitaman's manga list. I was curious, as I saw some manga that was not labeled "Hentai." After looking at the list, I noticed that there were several manga listed as "Seinen." Vitaman's demographic is hentai, not seinen.

Also, the best source I can give you is ja.wikipedia at most. Sorry about that. I also have a link with an Amazon.co.jp search. Amazon does not let you see the image unless you click it and also says 18+.
http://tinyurl.com/7ketj5m
http://ja.wikipedia.org/wiki/%E3%83%93%E3%82%BF%E3%83%9E%E3%83%B3
Wikipedia labels it as an "成人向け漫画雑誌," which basically is an adult magazine. (hentai simply put)

What are you talking about? the "成人向け漫画雑誌," label is pretty common in seinen magazines, like Young Comic: http://ja.wikipedia.org/wiki/%E3%83%A4%E3%83%B3%E3%82%B0%E3%82%B3%E3%83%9F%E3%83%83%E3%82%AF
From the English Wikipedia: Young Comic is a long-running Japanese seinen manga magazine started in 1967 as a magazine for teenage boys competing with titles like Manga Action and Big Comic. Today it targets a young adult male audience. It is published monthly by Shonen Gahosha as a sister publication to Young King.

"成人向け漫画雑誌," is not synonymous of "hentai", for God's sake. That is simply telling you that the magazine is aimed at adults, which is precisely what seinen magazines do. Those mangas serialized in Vitaman are pretty common in Seinen magazines. For example, Young Animal serializes Futari Ecchi which has those "Borderline Hentai" scenes in every chapter, so are you going to label that manga as "hentai"? If so, you should also label all manga serialized in that magazine like that, so Berserk, Amagami and Air Master, according to your logic, should also be labeled as "hentai"...

Ah, and those "warnings" that you got in Amazon are ridiculous. For example, you only get them when you want to search for the magazine, but if you search for the tankoubon of "Glass no Megami" ガラスの女神, a manga serialized in Vitaman, you get no warning whatsoever:
http://www.amazon.co.jp/s/ref=nb_sb_noss?__mk_ja_JP=%83J%83%5E%83J%83i&url=search-alias%3Dstripbooks&field-keywords=%83K%83%89%83X%82%CC%8F%97%90_&x=0&y=0

or Velvet Kiss ベルベット・キス
http://www.amazon.co.jp/s/ref=nb_sb_noss?__mk_ja_JP=%83J%83%5E%83J%83i&url=search-alias%3Dstripbooks&field-keywords=%83x%83%8B%83x%83b%83g%81E%83L%83X&x=0&y=0

or any other manga serialized in Vitaman. In short: Amazon is completely and utterly useless to determine if a manga is "hentai" or not and therefore should not be used to do so.
If something, there are tons of seinen mangas incorrectly labeled as "hentai" like the majority of manga serialized in Vitaman and that should be corrected.

Quote from JakeeDylan

Today I was looking at Vitaman's manga list. I was curious, as I saw some manga that was not labeled "Hentai." After looking at the list, I noticed that there were several manga listed as "Seinen." Vitaman's demographic is hentai, not seinen... Although this isn't a very important matter, I'm curious as to why this is like so. Not all of the titles in Vitaman have hentai as their genre because some of them, despite having adult/18+ content, don't meet the definition of hentai that is used at MU: "Adult sexual content in an illustrated form where the FOCUS of the manga is placed on sexually graphic acts." (You can see all the definitions on the genres page.)

Also, as far as I can tell from looking at Takeshobo's website and from the series running in the magazine, the demographic of Vitaman is indeed seinen. Remember that "seinen" is a demographic describing works that are created for late teen-twenties age males; that means, the hentai and adult genres could be said to fall under the seinen demographic. (They could fall under another demographic too, for example josei also has some adult titles, but my point is that "seinen" doesn't *exclude *"hentai" or "adult"--a series or magazine can be adult and seinen at the same time, like Vitaman is.) So it is okay for a magazine to have some adult titles or even some hentai titles, or to have adult as its main genre/theme, yet still be classified as seinen magazine. (For example, it is fine to call Kindai Maajan a "mahjong magazine," since that is its main theme, but its demographic is still seinen.) As far as I know, anyway. I hope that helps clear things up.

Manga Cover Database

Three years later, it seems like all the Vitaman titles got changed to Hentai. Is this intentional?