How to determine a quality backlink?

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Back links are links from other websites to your website – is the most effective way to improve your rankings.
Consider these factors for quality back links :

- Link sholud be from a relevant source
- Link sholud be from a trusted source
- Exact/phrase/synonym match anchor link text used
- On a page with PageRank
- Next to backlinks to authority websites
- From a different source to your existing backlinks
- Not reciprocal
- Few other backlinks on the page
- Not easily acquired