Why Canonical URLs Matter for SEO
A canonical URL tells search engines which version of a page is the "master" when multiple URLs serve similar content. Without it, signals can split across duplicates, hurting rankings. Set it with <link rel="canonical" href="..." /> or HTTP headers.
Canonical vs Redirects
Canonical tells search engines which URL to index, but both URLs remain accessible. Redirects (301/302) send users and crawlers to a different URL and consolidate everything. Use redirects when you want to retire a URL; use canonicals when multiple URLs should stay live but point to one preferred version.
Canonical vs Noindex
Canonical says "index this URL (or the canonical one) but treat these as duplicates." Noindex says "do not index this page at all." Use noindex for login pages, thank-you pages, or content you never want in search. Use canonical for duplicate or near-duplicate content you do want indexed under one URL.
Cross-Domain Canonicals & Common Mistakes
Cross-domain canonicals are allowed (e.g. syndicated articles pointing to the original). Common mistakes: canonical with tracking params, mixing www/non-www, relative URLs, canonicalizing paginated pages to page 1.
Ecommerce: Filters, Pagination, Variants
For product filters and pagination, each URL should canonical to itself (not to page 1). Use rel="prev"/rel="next" for pagination. For product variants (e.g. color/size), choose one URL as canonical and point variants to it.
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