see: http://www.ehow.com/about_4841771_bank-account-transfers.htm...

How Do Transfers Between Banks Work

The bank you are transferring money from needs to have an account, called a reciprocal account, with the recipient bank. Now, say you need to transfer $100. The bank you are transferring money to deposits this amount to the account you specified. However, it will take this $100 from the sending bank's reciprocal account. If the banks don't have reciprocal account numbers, then they use a third party, called a correspondent bank, that each of the banks has a reciprocal account with.