When Google finds similar URLs that are appropriate for specific devices (AMP pages, desktop browser, mobile browser), or marked as translations of the same page, Google might mark them as alternates for that device type, or language. Alternate versions are served when appropriate, based on the language or device of the user. Pages can be explicitly marked as alternates by the page creator using hreflang or rel=alternate tags or in a sitemap, or Google can decide that an unmarked duplicate is a good alternate URL. Alternates are a subset of duplicate URLs.
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