The banking book and trading book are two distinct portfolios that banks use to categorise their financial assets. Understanding this distinction matters because it determines how much capital a bank must hold against those assets. It also affects how gains and losses flow through to the bank’s financial statements and how positions are managed day to day.
The banking book contains assets that a bank intends to hold until maturity. These are typically customer loans, mortgages, and retail deposits. The bank expects to earn income from interest payments over the life of these instruments rather than from price movements. Credit risk is the primary concern here. The question is whether borrowers will repay their debts. While market values may fluctuate, this matters less if the bank plans to hold the asset to maturity and receive the contracted cash flows.
The trading book contains assets held for short-term trading. These include securities, derivatives, and positions taken to profit from price movements or to facilitate client transactions. Market risk is the dominant concern. Values can change rapidly based on interest rates, equity prices, foreign exchange rates, and credit spreads. Trading book positions are typically marked to market. Daily profits and losses flow through to the profit and loss account. Traders actively manage these positions.
Why does this matter for capital requirements?
Different risks require different treatments. Banking book assets face credit risk capital charges based on the probability of default and loss given default. The focus is on whether the borrower will repay. Trading book assets face market risk capital charges based on potential losses from adverse price movements. The focus is on how much value could be lost over a short horizon if markets move against the bank.
Historically, this created opportunities for regulatory arbitrage. Banks could sometimes reduce their capital requirements by classifying instruments in whichever book attracted lower charges. An instrument that would attract a heavy credit risk charge in the banking book might attract a lighter market risk charge in the trading book if volatility appeared low. The 2008 financial crisis exposed weaknesses in this approach. Some banks held complex instruments in the trading book with capital charges that proved far too low for the actual risks involved. When markets froze, these positions could not be exited and losses far exceeded the capital held against them.
What changed under Basel III?
The Basel Committee introduced the Fundamental Review of the Trading Book to address these weaknesses. The new framework creates a stricter boundary between the two books. Banks must now demonstrate genuine trading intent for assets placed in the trading book. Certain instruments are presumptively assigned to one book or the other. Moving assets between books requires regulatory approval and may trigger additional capital charges. The days of choosing the more favourable book for capital purposes are ending.
The criteria for trading intent are now more prescriptive. Instruments must be free of legal impediments to trading. The bank must have the capability to manage the position on a trading desk with appropriate risk management infrastructure. There must be documented policies and procedures governing the trading book. Positions must be fair valued daily with changes flowing through the profit and loss account.
Certain instruments are presumptively in the trading book. These include positions arising from market-making and from the execution of client orders. Certain others are presumptively in the banking book. Unlisted equities and equity investments in funds that cannot be valued daily belong in the banking book. So do instruments held for securitisation purposes. Banks must justify any departure from these presumptions.
UK and EU approaches
Both the PRA and EBA are implementing these Basel standards through their respective frameworks. The UK is implementing the trading book boundary rules from January 2027 as part of Basel 3.1. The EU originally planned implementation from January 2025 but has delayed the market risk elements to January 2026.
Both regulators issued no-action letters acknowledging that applying the boundary provisions before the full market risk framework could create operational difficulties for firms. The EBA extended this position in August 2024 and again for CRR3 provisions. Banks should not have to implement the boundary twice.
The core principle remains consistent across jurisdictions. Banks should not be able to choose their book allocation based on capital optimisation. The allocation should reflect genuine business purpose and risk management practices. Supervisors retain powers to require reclassification where they consider the allocation inappropriate for the actual business conducted.
