Market Mechanics: How Prices Are Set

Traders in New York, algorithms in Chicago, and a nervous retiree on her phone all help set the same stock price—down to the cent—at the same moment. If no one “decides” the price, yet everyone moves it, who’s actually in charge of what something is worth?

Roughly 80–85% of U.S. stock trades are now fired off by algorithms, not humans clicking “buy” or “sell.” Yet the price you see still looks like a single, simple number—as if the market quietly agreed on it. In reality, that number is the outcome of thousands of tiny negotiations happening every second inside electronic order books, where buy and sell orders line up and collide.

As new information hits—earnings surprises, economic data, a sudden spike in the VIX “fear gauge”—those lines of orders rearrange. Some traders race to pull orders, others rush to add new ones, and market makers step in to keep trading flowing, bound by rules that actually require them to quote prices a minimum share of the day.