It is a type of a business model of brokers.

A-book

A-book broker is referred as STP or ECN brokers.

It is the broker who sends all the orders from traders to the real market and doesn’t take a risk.

B-book

B-book broker is also referred as a market maker, but not necessarily.

It is the type of a broker who hedge clients’ trades.

It’s been known that more than 80% of traders lose their funds in trading, so brokers are making money by trading contrarily.

The B-book brokers usually make money if traders lose money.

A-book or B-book?

Of curse, the B-book type of model is popular among brokers, and normally A-book broker do turn into a B-book broker as soon as they acquire specialized program to hedge the trades.

MT4, MT5 and cTrader allow brokers to process trades with both A-book and B-book models.