What it means
A bank owns two kinds of things: loans it made to hold, and positions it bought to sell. The second pile is the trading book, and regulators treat it as a different animal.
The boundary is intent: instruments held with trading intent, for short-term resale or to profit from price moves, belong in the trading book; customer loans held to maturity live in the banking book. The accounting differs with the intent: trading book positions are marked to market through profit and loss daily, while banking book assets largely sit at amortised cost.
Capital rules differ accordingly: the Basel market risk framework, built by the Basel Committee and detailed in its market risk standards, sizes capital against the trading book's daily price swings. The boundary is also a temptation: booking a position where capital is cheaper can arbitrage the rulebook, which is why supervisors police the line and force documented intent.
History wrote the rules in scar tissue: the 1996 Market Risk Amendment first imposed models-based capital on trading books, and the post-crisis Fundamental Review of the Trading Book rebuilt the framework after 2008 exposed its gaps. The desk-level consequence is daily honesty: a trading book cannot hide a losing position in a drawer, because every evening's valuation feeds the bank's reported results.
For a non-finance reader, the trading book is the shop's display window rather than its storeroom: everything in it is for sale, repriced nightly, and its losses arrive immediately. The distinction shapes whole institutions.
Treasury departments, loan officers, and credit committees run the banking book, while traders, market makers, and market risk officers run the trading book, each with its own systems and controls. Compensation, accounting, and capital all follow the boundary.
In practice
Real-world examples.
Example
A desk parks illiquid notes in the trading book, flattering marks until a boundary review. The review asks for evidence of active two-way pricing and exit plans, and the notes cannot supply either.
Example
Half the inventory fails the trading-intent test, changing capital and accounting in one stroke. The reclassified notes move to the banking book, where they are held at amortised cost and attract credit-risk capital.
Example
The fix is documented intent at purchase, monthly boundary attestations and independent valuation. A valuation team outside the desk reports prices directly to risk, so the traders cannot influence the daily marks.
Formula
Calculation
There is no single formula; the regime is that trading book positions are marked to market daily, and regulatory capital is set by market-risk models, standardised or internal-models approaches under the Basel framework, against the positions' potential losses. Supervisors require clear internal policies defining and policing the boundary between the two books.
Worked example: a fictional trading desk holds $10,000,000 of bonds, valued at 100% of face value on Monday evening. On Tuesday the market price falls to 98.5%, so the position is worth $10,000,000 x 98.5% = $9,850,000. The desk books a loss of $10,000,000 - $9,850,000 = $150,000 through profit and loss that evening, whether or not it sells. The same bonds held in the banking book at amortised cost would still be carried at $10,000,000 unless a credit loss were recognised, which shows why the boundary matters.Case study
Seen in the real world.
This case study is fictional and illustrative. A made-up bank's new markets chief inherits a boundary problem in her first month: a desk has been warehousing illiquid structured notes inside its trading book while quietly treating them as investments. The classification decides the capital, and the capital decides the bonus pool's arithmetic. Her review walks the desk through the rulebook's question, not the position's story: was each note bought with genuine trading intent, evidenced by active two-way pricing and exit plans, or parked where mark-to-market felt flattering? Half the inventory fails the test, and the reclassification meeting is tense, because moving the notes to the banking book changes their capital, their accounting, and the desk's reported performance in one stroke.
The regulator's annual review, arriving by coincidence the following quarter, praises exactly that cleanup and cites the internal policy upgrades her team wrote around it: documented intent at purchase, monthly boundary attestations, and an independent valuation unit the desks cannot influence. Years later, mentoring a risk committee, she summarises the whole framework in one sentence: hold-to-maturity assets can wait for their story to finish, and trading book positions must survive their story being priced tonight. Her onboarding lecture ends with the rule she considers the whole job: never let a position's address depend on where its story sounds best. Desks that internalise that rule survive both the auditors and the markets. The ones that shop for classifications eventually meet both at once.
Watch out
Common mistakes.
- Assuming the boundary is optional; supervisors require documented trading intent, and boundary arbitrage is an enforcement target.
- Believing mark-to-market is cosmetic; trading book losses hit capital immediately, which is precisely why the regime exists.
- Forgetting illiquidity risk; a hard-to-sell position in the trading book still needs daily prices, and FRTB added liquidity horizons to the capital math.
Questions
People also ask.
What is a bank's trading book?
The set of positions held with trading intent, for short-term resale or price profit, marked to market daily and capitalised under market-risk rules.
How does it differ from the banking book?
The banking book holds loans and assets intended to be held, generally at amortised cost, with credit-risk rather than market-risk capital treatment.
What is FRTB?
The Basel Committee's Fundamental Review of the Trading Book, the post-2008 rebuild of market risk capital standards.
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