This was asked and answered on Web Applications.SE: https://webapps.stackexchange.com/questions/30254/why-does-gmail-login-go-through-youtube-com https://webapp

This was asked and answered on Web Applications.SE: https://webapps.stackexchange.com/questions/30254/why-does-gmail-login-go-through-youtube-com https://webapps.stackexchange.com/questions/30254/why-does-g... > When you log into a Google site like GMail, Google also logs you into all your various accounts Google accounts - such as YouTube. In order to be logged into a site, Google sets a cookie in your browser. > This is fine for most of their properties which are on the google.com domain (mail.google.com, docs.google.com, etc). However, a website can only set and read a cookie for its own domain - google.com can't set a cookie for youtube.com. To get around this, as part of the login process Google will redirect you through accounts.youtube.com to log you into YouTube - and typically your country specific domain if you are outside the US (e.g. I get logged into google.com.au as well).