In the following video, Motley Fool financial analysts David Hanson and Matt Koppenheffer talk about which customers are the most important for big banks. David tells us why both individual retail banking consumers and giant corporate clients have shown reduced profitability in today's climate, and why the mid-sized commercial business clients are where banks can really see strong margins. David also tells investors how important it is to follow follow along with the bank you invest in, to be sure it is increasing its base of clients in this range.