Doorway pages are Web pages designed and built specifically to draw search engine visitors to your website. They are standalone pages designed only to act as doorways to your site. Doorway pages are a very bad idea for several reasons, though many SEO firms use them routinely.
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Originally this method is called as "clocking" in seo "Doorway pages" is an another name of it. In this method two different website are hosted on a single domain server. When user ask for the website he gets a site which is totally different from the site which search engine gets for crawl.

Doorway pages are generally large places of poor-quality pages where each page is improved for a particular key phrase or expression. In many situations, doorway pages are published to position for a particular expression and then direct customers to a single getaway.

Doorway pages means you have see visitor something different and crawler different crawling means Doorway pages.