Nofollow was originally meant to be used for “anywhere that users can add links by themselves.” Nofollow links do not pass PageRank. Therefore, using a nofollow link means that link won’t pass PageRank to another page. This means, on the one hand, that Google cannot penalize you for linking to a site that’s known to be a low-quality site.
When to Nofollow links:
1. Links in Comments or on Forums
2. Advertisements & Sponsored Links
3. Links in Press Releases
4. Paid Links
Allow Followed Links in These Cases:
1. Bylines of Authors for Guest Posters
2. Links to Interviewees
3. Links in Editorial Content
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