title: Automatic cookie single sign on on multiple domains - like google
body: <p>I don't understand how google achieve the following mechanism of single sign on:</p>
  
  <ol>
  <li>I login in gmail for example (I suppose this creates a cookie withmy authorization) </li>
  <li>I open a new tab and direct type the url of "youtube"</li>
  <li>Then I enter youtube logged in.</li>
  </ol>
  
  <p>How can this second site detect that I've already been logged in.
  They are different domains. Youtube can't read the cookie of Gmail.</p>
  
  <p>All the solutions I've read about Single sign on don't allow this. The client always ask permission to a central login app.
  In my example YouTube doesn't know I am the same user logged in Gmail (actually it does know, but I don't understand how)</p>
  
  <p>Note that I type the url of "youtube" by hand. I don't clic the youtube icon from the upper toolbar of gmail (In that case gmail may pass some auth params through the url for example).</p>