Here is a list of situations where Google require the webmaster to add a rel=”nofollow”. Failing to comply may lead to a Google penalty.
- A link to a website in exchange for goods and services
- A link to a website in exchange for money
- A link to the company’s mobile app on an app store
- Excessive cross-linking between two web pages
- Text advertisements
- Advertisement Image links
- Low-quality links in widgets
- Bookmark site links (external)
- Widely distributed links in the footers or templates of various sites
- Links in forum signatures
- Links in comment signatures
- Register/join link to prioritize the crawling of other links on the page (no penalty risk)
- Login link to prioritize the crawling of other links on the page (not a penalty risk)
- Untrusted content like content that you don’t endorse
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