C-book, defined
The term C-book — used interchangeably with hybrid book — describes how a broker allocates client risk when it does not want to commit entirely to passing everything out (A-Book) or keeping everything in (B-Book). Instead, a risk engine decides, order by order and client by client, which flow to hedge externally and which to internalise. The "C" is best read as a combination of the other two rather than a distinct venue or counterparty. In reality, almost every broker of scale runs some version of this, because a rigid all-A or all-B stance leaves either revenue or risk control on the table.
The defining feature is that routing is dynamic. A trade from one client on one instrument might be hedged into the market, while an otherwise identical trade from a different client is warehoused internally — because the two clients present different risk and profitability profiles. The C-book is the policy layer that makes those decisions consistently and at speed.
Recap: A-Book and B-Book
The hybrid model only makes sense against its two components. In an A-Book, the broker passes client risk to an external counterparty — a liquidity provider or venue — so the broker earns from a markup or commission and is not exposed to the client's profit or loss. In a B-Book, the broker retains client risk internally, becoming the effective counterparty; it earns the spread and the net result of that internal book, but carries the market risk of unhedged positions. A-Book trades alignment and lower risk against thinner per-trade margin; B-Book trades higher potential margin against real balance-sheet exposure. The full comparison lives in A-Book vs B-Book: how brokers manage risk, and the execution mechanisms behind each are covered in STP vs ECN vs market maker.
A-Book vs B-Book vs C-Book compared
Setting the three side by side clarifies why hybrids exist: the C-book is an attempt to capture the best of both while accepting the operational and governance cost of running them together.
| Dimension | A-Book | B-Book | C-Book / Hybrid |
|---|---|---|---|
| Counterparty to the client | External liquidity provider | The broker itself | Both, decided per order/client |
| Market risk held by broker | None (passed through) | Full (retained internally) | Partial, actively managed |
| Revenue source | Markup or commission on flow | Spread plus net trading result | Blend of both, by segment |
| Conflict of interest | Low | Structural | Present on internalised portion |
| Technology required | Bridge to external liquidity | Risk/dealing system | Risk engine + routing + bridge |
| Best-fit broker | Neutral-execution firms | Well-capitalised risk-takers | Established brokers at scale |
Structural comparison. The C-book column describes tendencies of a hybrid policy; exact behaviour depends on each broker's segmentation rules and risk limits.
How flow segmentation and routing decisions are made
The engine of a hybrid book is flow segmentation: classifying orders and clients so the system knows what to hedge and what to keep. Common inputs include:
- Client profitability. Consistently profitable clients — whose gains would come out of the broker's own book if internalised — are often routed A-Book so their winnings are the liquidity provider's exposure, not the broker's.
- Instrument and volatility. Fast, thin or news-sensitive instruments may be hedged externally to avoid outsized internal exposure, while liquid, well-behaved instruments are more comfortably warehoused.
- Trade size and net exposure. Large tickets, or flow that pushes the internal book past a net-position limit, are routed out to keep exposure within risk appetite.
- Aggregate book position. Because many clients trade both directions, the broker often nets internal flow first and hedges only the residual imbalance externally.
These rules are not set once and forgotten; they are monitored and adjusted as client behaviour and market conditions change. Good segmentation is what separates a disciplined hybrid from an under-hedged B-book wearing a hybrid label.
The technology required
A credible C-book is a technology proposition as much as a commercial one. At minimum it needs a risk engine that values positions and tracks net exposure in real time, a routing layer that applies the segmentation rules to each order, and a bridge that connects the platform to external liquidity so hedges can be placed quickly and reliably. The A-Book leg is only as good as the liquidity behind it: when the engine decides to hedge, it needs deep, dependable pricing on the other side. That is where a Prime of Prime relationship fits — aggregated tier 1 bank and non-bank liquidity delivered through one connection, so the hybrid's external leg is fast and consistent even under stress. The connectivity and execution stack has to keep pace, because a routing decision that cannot be hedged promptly quietly turns into unintended internal risk.
Conflict-of-interest and transparency considerations
The internalised portion of a hybrid book carries the same structural conflict as any B-book: on those trades, a client's loss is the broker's gain. Running a C-book does not remove that conflict; it confines it to a managed subset of flow. Responsible operators address this with hard risk limits, governance over how segmentation rules are set, and disclosure to clients and, where applicable, regulators about how orders may be handled. Segmentation should be driven by risk management, not by a desire to disadvantage clients — for example, routing decisions based on genuine exposure and profitability are defensible, whereas manipulating execution quality for internalised clients is not. Transparency about the model, and consistency between what is disclosed and what actually happens, is what keeps a hybrid book on the right side of that line.
Why most established brokers run hybrid
Left purely A-Book, a broker forgoes the spread and net revenue available on low-risk flow and depends entirely on external pricing and credit. Left purely B-Book, it shoulders the full market risk of every client position, including that of skilled and well-funded traders. The hybrid model is the pragmatic middle: internalise the flow it can safely retain, hedge the flow it should not, and adjust the boundary as conditions change. That is why the C-book is the default for most brokers past the earliest stage — and why the quality of the A-Book liquidity behind the hybrid is decisive. Prop and funded-trader firms apply the same logic to their own payout risk, as covered in how prop firms make money. The glossary defines the supporting terms, and what is an STP broker explains the pass-through leg in more detail.