Final answer:
The principle of effective speech structure that requires even development of all parts is balanced structure or unity. It ensures the introduction, body, and conclusion are thoroughly developed with a clear thesis, supported evidence, and seamless transitions.
Step-by-step explanation:
The principle of effective speech structure that suggests all major parts of a speech should receive appropriate development is commonly known as balanced structure or unity. This principle ensures that each part of the speech, such as the introduction, body, and conclusion, is given equal attention and detail to form a cohesive message. A speech with a balanced structure will have a clear introduction that sparks interest and smoothly transitions to a clear thesis, a body with well-developed main points supported by solid, sufficient evidence, and a conclusion that ties everything back together and provides insight.
To achieve this, speakers should ensure that their thesis and organization are clear throughout the speech, with paragraphs that are evenly developed with topic sentences. Transitions should be used to ensure coherence and the overall delivery should show strong oral fluency and an appeal to the audience's sympathies and intelligence.