After the collapse of Silicon Valley Bank this spring, a lot of the agita in regional banking has cooled off, but national banks are still reaping the benefits. Big banks posted pretty big profits Friday: JPMorgan Chase, Wells Fargo and Citigroup told investors they netted a combined $22 billion this past quarter.

The classic way banks make money — like, the way they teach it in Banking 101 — rests on three words: net interest income.

“What the bank is receiving in interest, and you subtract the interest going out,” said Mayra Rodríguez Valladares, managing principal at MRV Associates.

Banks charge higher interest rates on mortgages or commercial loans or credit cards than the interest they pay you on your checking and savings accounts. And when the Federal Reserve hikes rates from near zero to 5% in just over a year, that net interest margin actually gets bigger.

“Any of the new loans they’ve been originating the last two years, they get to charge more and more. And yet, there is a lag in how much and when the banks pay a higher rate,” Rodríguez Valladares said.

That lag is the biggest reason behind national banks’ profits this past quarter. The big banks know you’re probably not going to leave them even if you could get a slightly higher annual percentage yield elsewhere, and even if you do, they have tons of other depositors and income streams.

Smaller, regional banks though? Not so much, said Zander Yokum at CFRA Research.

“So when they lose deposits, it can actually stop their ability to fund loans. So because of that, they really don’t want to lose deposits, and therefore they pay a higher rate,” Yokum said.

Yokum said the fear-based flight out of regionals into national banks has subsided. But if you’re a bank that’s not too big to fail, you might be too small to be very profitable right now.

“Regional banks are definitely struggling. Concern obviously in the office space and commercial real estate arenas,” Yokum said.

Regional banks often specialize in commercial real estate lending, and demand for office space is still pretty weak.

But while regionals will eat those losses most directly, Kairong Xiao at Columbia Business School said the big banks may not be immune.

“Some of those loans gonna be packaged in the commercial real estate-backed mortgage-backed security,” Xiao said.

And Xiao said those commercial real estate mortgage-backed securities may be on some big banks’ balance sheets, waiting to be marked down.