There are only a handful of tools that look at topic relevance in a website. Google doesn't like websites that create content silos: it wants to see holistic tools: no more optimizing by keywords only.
This tool leverages the best of class SEMrush data to find the actual topic and keywords Google discovers with its LSI algorithm, Hummingbird. It's said by many that "relevance and relatedness" is now second only to links for high ranking, plus your users have a better experience.
What better way to discover related topics than by this process that lets you reverse engineer Google's AI on topics? Review collected by and hosted on G2.com.
API key from SEMrush required (no affiliation) BUT it only costs $15 and SEMrush is best of class for using this data. Review collected by and hosted on G2.com.
