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Tankobon / 単行本
Def: A bound volume containing one or more chapters from a serialized manga.
Manga are usually run in anthology magazines like Shonen Jump or Betsucomi, with one or two chapters per issue. (These anthologies run many series at a time.) After a certain amount of time, the chapters that were run in the magazine are sold as tankobon, containing an amount of chapters that can add up to more than 100 pages.
The amount of chapters in a tankobon depends on what kind of magazine the manga ran in. For example, monthly magazines have longer chapters with about 5-chaptered tankobon, but weekly magazines can have about 10 chapters in one tankobon.
An incentive to buying tankobon are extra mini-comics and comments from the author. When the manga is run in a magazine, there are side-panels used for advertisements. These ads are removed for the tankobon and authors often use the space for commentary, sketches, or a smaller comic. (Examples where this is often used are Beauty Pop, Ouran High School Host Club, Death Note.)
In Japan, tankobon are sold with a booksleeve. It will have cover art, and include the name of the publisher, and the magazine it ran/runs in. When sold in the U.S., tankobon usually don't include all of the same- they are sold as paperbacks, and will not include the anthology magazine (the one exception I know of are the manga uner Viz Media's Shonen Jump label, whose manga all ran in the anthology of the same name). Some Original-English-language manga (Dramacon, Off☆Beat) are not run in magazines, and are sold direct-to-tankobon. Some also show no chapter breaks, even in an ongoing series. The English-language YEN+ serializes a few American-drawn manga (Svetlana Schmakova's Nightschoool, James Patterson/NaRae Lee's Maximum Ride) in their monthly magazine, selling tankobon the same way they do in Japan.