Fans of Girl Genius (and this wiki) use the term canon (adjective: canonical) to refer to the body of work that is considered "almost certainly true" with respect to the ongoing main narrative — that is, the story about which the majority of the comics published Monday, Wednesday, and Friday at girlgeniusonline.com concern themselves.
Note well: the canon as defined here is currently incomplete, is probably inconsistent, and would seem inherently resistant to the admission of both completeness and consistency.
To maximize the utility of this article, the concrete list is presented first; qualifications follow concerning the use of this list and the material in the items.
- Graphic Novels: The story starting, roughly, on .
- Novels: Girl Genius Novelization
- Word of the Gods (WoG): Direct statements by the authors and descriptive elements such as cast lists and prologues (some of which are available for free on-line, some of which are included only in the printed works (or PDFs thereof)).
- Indirect Word of Gods: Sometimes the Word of God is relayed through other people, reports of with or by the authors. These are generally considered part of the Canon, with the caveat that the person who reports the information may be unreliable.
Although official material from or approved by the Professors, the following is not directly connected to the main narrative and thus not accepted as canon.
- Earlier published Girl Genius-based works. Would have been considered canon "when they were new."
Insane or out of sequence
- These apparently happen in the future (relative to the main narrative), but we don't know which future. (They can also be said to be taking place in the fictional present of the time they were written.)
Stories told about Agatha and her associates and written down in a very convincing manner, no matter their veracity:
- The forthcoming Girl Genius Sourcebook and Roleplaying Game, which is said to be, essentially, Secret Blueprints II.
- Kaja Foglio occasionally has referred to a Girl Genius bible which was created for use of those involved in producing a film version. A "bible", in entertainment industry terms, is the equivalent of "Word of God" in fandom — the base reference for the characters, situations and interactions, including those to be revealed in the course of the production, written by the creators.
The option to create the film expired without being exercised, and the bible is now outdated. It has not been released to the public and would be essentially pure spoiler, at least for the timeframe covered by the script (a spoiler that in many cases no longer spoils).
Use of canon
The safest, least controversial assumption is that what appears to happen in the frames of the main narrative actually did happen. As far as the wiki goes, when writing for main articles, the rule should be to take the facts, by this definition, at face value whenever possible.
Similar to the "what appears to happen, did happen" rule, it should go without saying that direct answers by Phil or Kaja to fans' questions are taken at face value. The authors are not expected to dissemble in real time.
Use of non-canon
Since it's difficult to obtain much common sense from the story itself, it's tempting to turn to related works for whatever they can supply. The common opinion is more-or-less that the non-canon works cannot be used as direct evidence when arguing a point. There are details that have shown up first in them that later became canonical.  Nevertheless, until such occurs, they will only be accepted as circumstantial evidence. (As usual, standards are relaxed on /Mad pages.)
Othar's Twitter was originally an exercise in collaborative writing among a group, but others dropped out soon and now Phil does it himself. As implied in Non-canonical above, this work has inspired some of the discussion about the meaning of "canon" as related to Girl Genius and consequent queries to the authors. Although it is clearly part of the Girl Genius canon in the literary sense of the word, in practice it is not "canonical" for use on this wiki.
Remember the cardinal rule: This is mad science.
Canon, which shows that this discussion is not limited to Girl Genius.