Final answer:
For organizing supporting materials in an outline to ensure no redundancy and that each point expresses one idea, the key principle is unity. Unity helps maintain focus and cohesion in the document's structure.
Step-by-step explanation:
When organizing supporting materials within your outline to ensure that each element expresses a single idea and to avoid overlapping themes or redundancy, the principle to follow is unity. Unity in an outline or essay ensures that all paragraphs and sections contribute to the overall argument or theme without straying into off-topic areas. By maintaining unity within an outline, the writer can develop each point fully and cohesively, ensuring that each paragraph builds upon the last and supports the overall thesis.