It’s the believability clause! Not that I have to be 100% accurate when I insert historical information into my novel however, when writing about events that occurred in the past it is important that your plot agrees with history as closely as possible. For example: there were no automobiles in 1776.
I wanted the character Moses to come from the city next to the plantation where Horace X the slave lived. If my goal was to just get the 50,000 words out of the this would not have been a problem. After doing some research I realized there could not possibly be a bustling city next to a cotton plantation. The graphics I discovered sent me back to the novel I was editing prior to writing this one since it is book one of this series.
I am not writing Historical Fiction, it is still important to have some idea of what happened during the era in which you are writing. This is THE BELIEVABILITY CLAUSE.