Plausibility

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Plausibility isn't the only thing that alternate history gets graded on, but it's one of the most important categories.
Plausibility has a weird relationship to possibility. As a general rule of thumb, the more unlikely things get, the less plausible the ATL is judged to be - but if you introduce something that's decisively NOT possible, it's more of a genre shift than a serious mark down. Once it's established that the ATL is an ASB story, the standards for plausibility change - very slightly, though: what's plausible in an ASB verse isn't far removed from what's plausible in a "normal" one.
Category:AH Tropes 101