Speaking of prediction markets, many people have heard of Polymarket and know its seemingly simple rule — YES plus NO always equals 1. But to truly understand the logic behind it, it's not that straightforward.

Today, I will explain the shared order book mechanism of Polymarket clearly.

If you look at the official documentation, you'll see explanations about price calculation. At first glance, it might be confusing, but don't worry — a simple example will make it clear.

**A torn one-dollar bill**

Someone might ask: if YES is priced at 0.7 and NO at 0.6, totaling 1.3, can't they be freely priced in a free market?

Wrong. Although it's a free market, YES and NO are not two independent stocks. They are actually two parts of the same one-dollar bill.

Think of it from a different perspective: Polymarket isn't selling a lottery ticket, but a future redemption voucher. The value of each voucher is essentially 1 dollar. The market tears this 1 dollar into two halves, one labeled YES and the other labeled NO.

On the settlement day, if the event actually occurs, the YES voucher is worth 1 dollar, and the NO voucher becomes 0. Conversely — if the event does not occur, the YES voucher becomes zero, and the NO voucher is worth 1 dollar. So:

Event occurs: 1 + 0 = 1
Event does not occur: 0 + 1 = 1

In any case, the total value of these two vouchers is always 1. That’s why, in an efficient market, the prices of YES and NO must sum to 1.