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Possible Duplicate:
Is there a usability reason for centering a website’s content area on a page?

For a website with fixed width (ex. Stack Exchange, Twitter UI) which shows content in a central location;

  1. What are the determining factors to use in order to decide whether to centrally align the main content like Stack Exchange (or Twitter) or ti align left like Google's search results page?
  2. What should be considered before making the decision? (Especially if the design suits both alignment choices)