Happy Path
When writing code (or documentation), the easiest case to consider is the 'happy path'.
The happy path is the case where no exceptions occur, and no special checks or caveats are required. The happy path is that special set of circumstances in which all the easy assumptions hold.
As a coder, or as a tester, or as a writer, or as a security expert, or as a politician, you spend the majority of your actual time, concerned with everything other than the happy path.