Quant Insider.io

How Banks Actually Organise Their Trading Books 🧵

Most people, even junior traders have no idea how a dealer bank actually structures its trading books.

This structure determines risk, P&amp;L, capital, limits, and even who gets paid bonuses.

Here’s the real breakdown 👇

External Books - The Only “Real” Risk

External books contain trades with:
•clients
•other banks
•exchanges/CCPs

Whatever sits here is true economic exposure.
This is what drives:
•VaR
•stress tests
•regulatory capital
•liquidity requirements

If it’s not in an external book, regulators don’t care  and neither should the firm.

Internal Books - The Hidden Plumbing

These are trades between desks inside the bank:
Sales → Trading
Exotics → Vanilla
Vanilla → Delta-one
Trading → XVA
Trading → Treasury

Internal books MUST net to zero across the firm.

If they don’t, your risk reporting is already broken.

Purpose:
•reassign risk to the right desk
•transfer P&amp;L fairly
•consolidate expertise (e.g., vanilla desk manages vega)
•save bid–ask by using internal liquidity before hitting the market

They change who owns the risk, not how much risk the firm has.

Dummy Books 

These exist solely for:
•testing new products
•testing valuation/P&amp;L changes
•running model upgrades
•verifying booking workflows

They should NEVER appear in:
•real P&amp;L
•VaR
•limits
•capital
•trader dashboards

If dummy books leak into risk?
Your entire risk system’s credibility collapses.

The Book Hierarchy Inside a Dealer Bank

All books roll up into a structured tree:

Global Markets
→ Equities
→ Derivatives
→ Index Options
→ Trader_A_Book

Risk &amp; P&amp;L are aggregated up this hierarchy for:
•desk limits
•business-unit limits
•firm-wide reporting
•CEO/CRO dashboards

This is why even the naming convention of a book matters.

Why This Structure Exists

Risk must sit with the desk best equipped to manage it

(Exotics handle path-dependent risk, Vanilla handles vega, Delta-one handles linear risk)

Regulators want visibility only on external exposure

Internal P&amp;L must reflect actual desk contribution, not noise

A clean book structure is the backbone of a professional trading operation.

The One Mistake Juniors Always Make

They think:

“If Sales trades with Trading, the firm has two trades.”

Wrong.

Internal trades wash out.
The firm’s only exposure is to the external counterparty.

Miss this concept and you’ll never understand how banks manage risk.

Summary

The entire trading organisation rests on three book types:

External = real market/credit risk

Internal = risk transfer inside the bank

Dummy = testing only

This structure decides:
•who owns what
•who gets paid
•how much capital the bank must hold
•and ultimately, the bank’s survival

Yet most people on the outside never see this.