YES and NO are one book

One contract, two ways to hold it

A YES contract pays $1 if the event happens and nothing if it does not. A NO contract pays $1 if it does not happen. Hold one of each and you are paid exactly $1 whatever happens.

So buying NO at q is the same position as selling YES at 1 − q. The exchange keeps a single book: a bid to buy YES at 47 cents is also an offer to sell NO at 53 cents.

The identity

It always holds, because the NO ask is one dollar minus the YES bid. An example:

The YES spread is 2 cents. YES ask plus NO ask is $0.49 + $0.53 = $1.02, which is $1 plus that same 2 cents. The NO bid of $0.51 is simply one dollar minus the YES ask.

Buying both sides under a dollar is impossible

A common idea is to buy YES and NO together for less than $1 and collect the dollar at settlement. On a single book it cannot work. Buying both at the ask costs 1 + spread, plus a taker fee on each side, for a payout of exactly $1. The loss is the spread plus two fees, every time.

What it means for reading the book

• Depth on the NO side is depth on the YES side, priced from the other end. It is not extra liquidity.
• A tight YES spread means an equally tight NO spread. There is only one spread.
• Whether you click YES or NO, you are trading against the same resting orders.