What should a formal outline have?

A formal outline should include a clear hierarchy of ideas, typically organized into main topics and subtopics, using Roman numerals for main points and letters or numbers for supporting details. Each level of the outline should be indented to show the relationship between ideas. Additionally, it should be concise and use parallel structure for consistency, ensuring that each point reflects the same grammatical form. Finally, it is essential to include a title or thesis statement at the top to convey the overall purpose of the outline.