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If you have a very small blog, with a limited number of posts on a narrow topic range, this perhaps doesn't apply. If you are bigger than that and cover a range of topics, a multilevel structure keeps things organized, for you, your readers, and for search engines. You can also take advantage of internal linking, organizing sub-topics under topics both so that readers can find related articles that might interest them and so that search engines can see how your posts are related, and what concepts are relevant. [A post titled "Shingles" would mean one thing under health-news, and another under building-materials.]