My column in today’s Keys Weekly:

Q. What exactly is causing all these banks to fail?

A. Let’s review the business model of a bank. If you have a checking account or a car loan, you probably already know the basics.

A bank takes in money from one group of people, depositors, lends it to another group, borrowers, and profits from the difference. If a bank borrows money from a depositor at 3 percent and lends it out at 6 percent, the bank has earned a 3 percent spread, or net interest income.

Most banks also make money from fees and other services, typically called noninterest income, which when combined with net interest income comprises the bank’s net revenues.

Much more than other businesses, a bank’s revenue and profits are tied to its balance sheet. You’ll recall a balance sheet is the financial statement that summarizes a company’s assets, liabilities and equity at a specific point in time. The balance sheet shows what is owned, what is owed, and what is left over.

On the asset side of a bank’s balance sheet, you’ll find loans and investments, and on the liabilities side you’ll see deposits and borrowings. One important point to understand about a balance sheet is that while the value of assets may change, liabilities are fixed. Since the amount of assets is always equal to liabilities plus equity, any change in the bank’s assets will be reflected in its equity, too.

If the Bank of Margaritaville had $50 million in both assets and liabilities, but no equity, then a small decline in the value of those assets would mean that the bank could not meet its debts. The bank would become insolvent.

So a bank’s equity is a critical cushion for both depositors and bank shareholders.

What sorts of things would cause that equity cushion to disappear? Bad loans, which effectively reduce the amount of assets on a bank’s balance sheet and therefore its equity, too. A constant barrage of bad loans could eventually drain the equity right out of a bank – if the regulators don’t step in and seize the bank first.