Off-page SEO: backlinks and domain authority
Everything covered through part 15 has been fully within a site owner's direct control — content, structure, technical setup. A backlink — a link from another site pointing to this one — is the first major ranking factor that fundamentally isn't: it depends on someone else, independently, deciding a page is worth linking to.
Why a backlink functions as a vote of confidence
The original insight behind Google's PageRank algorithm — still conceptually underlying how links are weighed today, even as the system has grown far more sophisticated — treats a link as an implicit endorsement: another site's owner chose to point to this specific page, out of the entire internet, as worth referencing. A page earning links from many separate, independent sources is treated as more likely to be genuinely valuable than one with none, on the reasoning that real value tends to get cited.
Not all backlinks carry equal weight
A link from an established, authoritative, topically-relevant site (a well-known publication in the same field) carries meaningfully more weight than the same link from a low-quality, unrelated, or spammy site — and past a point, links from genuinely low-quality sources can actively hurt rather than simply doing nothing, which is exactly the pattern part 21 covers under manipulative link schemes search engines actively penalize.
dofollow vs. nofollow
<!-- Passes ranking signal (the default for any plain link) -->
<a href="https://example.com/article">Read the full article</a>
<!-- Explicitly does not pass ranking signal -->
<a href="https://example.com/article" rel="nofollow">Read the full article</a>
<!-- Specifically for paid or sponsored placements -->
<a href="https://example.com/product" rel="sponsored">Learn more</a>
<!-- Specifically for user-generated content, like a comment section -->
<a href="https://example.com" rel="ugc">Visitor's link</a>A plain <a href> with no rel attribute passes ranking signal by default — that's the "vote" from part one of this page. rel="nofollow" explicitly withholds it; rel="sponsored" and rel="ugc" are more specific variants Google introduced to distinguish why a link shouldn't pass signal — a paid placement versus an uncontrolled comment section, respectively — since those carry different implications for a search engine trying to detect manipulation.
Domain authority: a third-party metric, not a Google one
"Domain Authority" (a specific metric from Moz) and similar scores from other SEO tools (Ahrefs' "Domain Rating") are third-party estimates of a domain's overall link-based strength — useful as a rough, comparative signal, but not a number Google itself publishes, uses directly, or has ever confirmed corresponds precisely to how it actually weighs authority internally. Treat these scores as a directional heuristic for comparing sites, not as a literal ranking factor to optimize for its own sake.
Anchor text on an inbound link matters too
The same anchor-text principle from part 5's internal linking discussion applies to backlinks, and arguably matters more: a backlink using descriptive, relevant anchor text ("Python requests library tutorial") reinforces a page's topic more directly than the same link using generic text ("this page") or, worse, the bare URL with no descriptive text at all.
Chasing backlink quantity as the primary goal, from any source willing to link — low-quality directories, link farms, irrelevant sites. A smaller number of genuinely relevant, authoritative backlinks consistently outperforms a large number of low-quality ones, and past a point, an unnatural pattern of low-quality links is itself a negative signal search engines are specifically built to detect.
Next: link building — the actual, legitimate strategies for earning the kind of backlinks this part just described, rather than manufacturing them.