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Structured data addresses a fundamental problem: HTML tells machines what is displayed, but not what it means. A heading could be a product name, an article author, or the answer to a question. Schema.org markup adds this semantic layer — machine-readable, invisible to users.
JSON-LD: the recommended format
There are three ways to implement structured data (JSON-LD, Microdata, RDFa), but Google explicitly recommends JSON-LD for its ease of maintenance and flexibility. The JSON-LD block is embedded in a <script type="application/ld+json"> tag, typically in the <head> or at the bottom of the <body>. It can be updated without touching the page HTML.
The schemas with the most impact
Impact varies by page type and objective. For classic SEO visibility: FAQPage (triggers expandable rich snippets), HowTo (displays steps directly in the SERP), Product (reviews, price, availability), Article with datePublished and author. For GEO visibility: DefinedTerm and DefinedTermSet allow LLMs to precisely identify and cite glossary definitions, ItemList structures ordered extractable lists, and SpeakableSpecification flags passages optimized for voice reading.
Structured data and LLMs
LLMs trained on web corpora have integrated Schema.org logic into their content understanding. Content explicitly marked up with a DefinedTerm is more likely to be cited precisely and correctly than content whose nature must be inferred from context. Structured data reduces ambiguity for machines — which reduces hallucination risk and increases citation fidelity.
