A bank's securities portfolio is not there to make money. It is there to hold liquidity that earns something, to soak up deposits the loan book cannot use, to manage the interest-rate position of the whole balance sheet, and to be pledgeable. Judged as an investment portfolio it looks absurdly conservative. Judged as a liquidity instrument with a capital charge attached it usually looks about right.
Three rules shape everything a bank holds. Capital: risk-weighted assets drive the denominator of every ratio the regulator cares about, and a government bond at a zero risk weight consumes no capital while a corporate bond does. Liquidity: high-quality liquid assets are defined by regulation, and a security that does not qualify does not do the job the portfolio exists for. Accounting: whether a security sits in held to maturity or available for sale decides whether its price changes reach reported equity and, sometimes, regulatory capital.
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How do held to maturity and available for sale differ?
That third rule produces disasters, because it looks like a filing choice and behaves like a strategy. Under US generally accepted accounting principles an available-for-sale security is carried at fair value, with unrealised gains and losses running through accumulated other comprehensive income, while a held-to-maturity security is carried at amortised cost with fair value disclosed in a footnote. Regulatory capital adds a wrinkle. US banking organisations outside the advanced approaches may make a one-time election to opt out of including most of those amounts in common equity tier 1 capital. (The election and the risk weights sit in the US capital rule at 12 CFR part 3, 217 or 324 depending on the agency; check the current text and the bank's own disclosed election.)
Three treatments therefore exist for the same bond at the same price. Available for sale without the opt-out puts the after-tax mark through both accounting equity and regulatory capital. Available for sale with the opt-out puts it through accounting equity alone. Held to maturity puts it through neither, leaving it in a footnote. Worked Example 4 runs a $20,000 million Treasury book through all three after a 200 basis point rise, and the spread between the best and worst capital ratios is 2.54 points on an identical portfolio.
What is the held-to-maturity trap?
Then it springs the trap, which is what makes this more than an accounting curiosity. The held-to-maturity classification protects the ratio right up to the moment the bank needs the money, and then it protects nothing: raising cash against a deposit outflow forces a sale at the marked price, and selling out of the category calls its stated intent into question for everything left inside it. In the worked example the ratio ends up within a rounding error of where the marked available-for-sale case sat from the first day.
The classification never changed the economics. It changed when the economics became visible, which made the bank's own crisis arrive as news rather than as a trend.
This is not hypothetical. Silicon Valley Bank publicly announced on 8 March 2023 that it had sold approximately $21 billion of available-for-sale securities at an after-tax loss of about $1.8 billion; the deposit run and the bank's closure followed within two days. (Check the figures against the company's own filing of that date. They are cited here as a dated public-record fact and nothing further is inferred from them.) The general point stands without the example: for a bank, an unrealised loss is a liquidity problem wearing an accounting costume, and the size of the costume is the size of the deposit base that can leave.