I'm not a finance guy but I took a finance class once... I believe that all banks are required to hold a minimum reserve that is set by the Fed. Each day banks

I'm not a finance guy but I took a finance class once... I believe that all banks are required to hold a minimum reserve that is set by the Fed. Each day banks with assets greater than needed "sell" their reserves to banks with less via T-notes. This is all due to regulation, banks on their own wouldn't participate in this because it costs the banks with low reserves money but it keeps banks from disappearing overnight like MtGox. To be fair though Gox was an exchange and not a bank and that is a bit different Edit: Wow bad spelling..