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You are telling a story, not writing a manual. Everything goes in the story. The order in which it occurs in the story is the order in which it matters to the story. There are two ways to introduc...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/24887 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
You are telling a story, not writing a manual. Everything goes in the story. The order in which it occurs in the story is the order in which it matters to the story. There are two ways to introduce background material into a story. You can tell the reader yourself as narrator, or you can have one character tell another character. Both approaches are entirely legitimate, but they presume a different narrative style. If you choose to do have one character tell another, you need to do something to make it convincing that the characters are actually having this conversation. This can be bogus, as in every episode of every CSI show ever, in which one CSI explains the experiment to another CSI (who presumably would know all about it) as a means to tell it to the viewer. This is utterly bogus but is somehow made acceptable to the audience as part of the overall stylized nature of the show. Nothing is authentic about these shows, but they sell their conventions so relentlessly, and with so much glamour, that you half-willingly accept them. You can also do something like the Council of Elrond in which you contrive a reason for people of different backgrounds to be in one place to make an important decision and have each of them in turn explain part of the background to the others. The thing to note about this is that such explanations have to be in character -- the thing that this character would say at this moment to serve their personal agenda -- or they are not convincing and sound like an infodump.