The basic idea of consistency on websites is simple: when people move around from page to page in your website, similar information or features should be presented in similar ways. A consistent user interface is one that presents the same options in the same way on all of the pages of the site.
There is also a good case for being consistent across the Internet. For example, when people have been looking at your site for a while, and want to know more about your company, studies show that most people look for a piece of text that says "About Us." Is there something about these words that are inherently superior to any other words you might use - like "Company Profile" or "Corporate Mission?" No - it's just that the words "About Us" were used from the start, and used most often. Now, it is a de facto standard for consistency across the Internet.
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