What it means
The core idea is a live inventory. Every purchase, sale, hedge and derivative sits in the book, and the book's value is recalculated continuously as prices move.
A book has a direction. If long positions outweigh short positions the book is net long and profits when prices rise, and if shorts dominate it is net short and profits when prices fall.
Traders describe the book by its size in two ways. Gross exposure adds the long and short positions together and shows how much activity is on, while net exposure subtracts one from the other and shows which way the desk is actually pointing.
Risk managers care about the book, not the individual trades. A single large position may look alarming until you see the offsetting hedge sitting next to it, which is why limits are set at book level rather than trade by trade.
The same word travels well beyond trading floors. An insurer talks about its book of business meaning the portfolio of policies it has written, and a lender talks about its loan book meaning all outstanding advances.
Be careful not to blur trading books with accounting book value. The first is a live, marked-to-market view of positions, while the second is a historical cost figure recorded in the ledger and updated only at reporting dates.
In practice
Real-world examples.
Example
A commodities trader ends the week net long 400 contracts and explains to the risk committee that the book is positioned for a supply squeeze. The committee focuses on the net figure and the size of the hedges rather than the individual trades.
Example
A regional bank describes its loan book as $1,200,000,000 with an average maturity of four years. Analysts use that description to estimate how quickly the bank's interest income will reprice if rates change.
Example
An insurance broker sells its book of business to a larger competitor on retirement. What changes hands is the renewal rights to the client portfolio, valued as a multiple of annual commission income.
Formula
Calculation
Net book profit and loss = (mark-to-market value of longs - cost of longs) + (proceeds of shorts - cost to close shorts)
A trading desk holds a long position of 200,000 shares bought at an average cost of $18.40, giving a cost of 200,000 x $18.40 = $3,680,000. The shares are now marked at $19.75, so the position is worth 200,000 x $19.75 = $3,950,000 and shows an unrealised gain of $3,950,000 - $3,680,000 = $270,000.
The same book contains a hedge: a short position of 50,000 units of an index instrument sold at $30.00 and now trading at $31.00. Because the price rose, the short loses 50,000 x ($31.00 - $30.00) = $50,000.
The net book position is $270,000 - $50,000 = $220,000 of unrealised profit. Looking at either leg alone would give a misleading picture, which is precisely why performance is measured across the whole book.Case study
Seen in the real world.
The following is an illustrative and entirely fictional example. Ashcombe Securities, an invented brokerage, ran a corporate bond desk whose head reported only net exposure to the board. The book showed $40,000,000 of long positions and $34,000,000 of shorts, so the net figure of $6,000,000 looked modest and nobody asked further questions.
The trouble was that the longs and shorts were not in comparable instruments. The longs were in thinly traded bonds from smaller issuers, while the shorts were in liquid government securities used as a rough hedge, so the two legs did not move together when credit spreads widened.
When spreads gapped out, the fictional desk lost far more on the longs than it made on the shorts, and the board discovered that a $6,000,000 net position sat on top of $74,000,000 of gross exposure. The firm rewrote its reporting so every book showed gross and net side by side, along with a note on how closely the hedges actually tracked the positions they covered.
Watch out
Common mistakes.
- Reading net exposure as the whole story, when a small net figure can hide very large offsetting positions that behave differently under stress.
- Treating a trading book and book value as the same concept, when one is a live position record and the other is an accounting measure.
- Assuming a hedged book carries no risk, when imperfect hedges leave basis risk that only shows up when markets move sharply.
Questions
People also ask.
What does it mean to run a book?
It means managing a portfolio of positions actively, setting the balance between longs and shorts and keeping the risk within agreed limits.
Is a book always about trading?
No. Lenders speak of a loan book and insurers of a book of business, both meaning the total portfolio of contracts they hold.
What is the difference between gross and net exposure?
Gross adds long and short positions together to show total activity, while net subtracts shorts from longs to show directional bias.
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