Easy means should be provided for users to specify required format control features (e.g., margin and tab settings) during text entry/editing.
Additional Information:
Required format features will vary depending on the application. The intent of this guideline is that all required format features should be easy to control. Any format features that are provided but are optional for the user's task should not be made easy to use at the expense of required format features. One convenient method of margin and tab control is to allow users to mark settings on a displayed "ruler" that extends the width of a page and is continuously displayed at the top of the screen.