Use multiple sign-in with Google Docs
If you are using Google Docs, and you're signed in to multiple accounts, visiting http://docs.google.com will automatically sign you in to your default account.
If you use Google Apps, going to your Google Apps domain's URL (e.g. http://docs.google.com/a/my-domain.com, or possibly http://docs.my-domain.com) will automatically sign you in to your Docs for your @my-domain.com account. Multiple sign-in is particularly useful for Google Apps accounts since you may sign into multiple Google Apps domains.
If you are using multiple sign-in with Google Docs, you can only sign in to Google Docs with one Google Account at a time. However, if you have a Google Apps @my-domain account, you will be able to sign in to Docs with a Google Apps account and a Google Account.
The first Google Account you use to sign in to Docs will be the account you'll always use to access Google Docs. If you sign in to another Google Account (that isn't a Google Apps account) using multiple sign-in, you won't be able to use Google Docs with that account.
Even though you may notice additional Google Accounts in the drop-down menu when you click your name or email address at the top of the page, you can only use your default account or any Google Apps accounts you've signed in to initially to access Docs. Learn more.