A list of pages on a website that search engines should index. Written in extensible markup language (XML), both humans and computers can understand a sitemap XML. In addition to page URLs, they include modification dates so search engines can tell whether or not a page has been crawled since it was updated. Sitemap XML- and robots.txt-related search engine optimization (SEO) tactics approach indexing from different sides of the same coin. Sitemap XML says “yes” while robots.txt says “no.”