The mechanics of a slot machine

Slots are king.

Across the country, casino slot machines bring in at least $20 billion a year, raking in more cash than movie tickets, retail book sales and specialty coffee, according to industry reports.

In Illinois, 9,946 slot machines brought in 88 percent of the revenue at the state's nine casinos last year.

Meanwhile, the Chicago region and northwest Indiana make up the third-largest casino market in the country, trailing only Las Vegas and Atlantic City.

Slot players lost $1.7 billion at Illinois casinos last year.

At the nerve center of these prolific moneymakers sits a computer chip called a Random Number Generator, or RNG for short.

This chip continually spits out numbers, and each string of numbers corresponds to a certain win or a loss. This is displayed on the slot screen as the final combination of virtual reels or on a mechanical machine by how the actual reels align.

Where the graphics or symbols will fall is determined at the moment the slot arm or button is engaged.

The RNG is programmed to produce a certain hold for the casino over time. This is called the hold percentage, or inversely the payout percentage.

Under Illinois law, all slots have to be programmed to hold no more than 20 percent, or 20 cents from every dollar gambled over time. State regulators from the Illinois Gaming Board inspect the chips and slots before they go on a casino floor and do random spot checks after that.

The Daily Herald calculated the data for this project using the state gaming board's annual reports. The records outline the number of slot machines at each casino that fall into a particular denomination, such as penny or $1. The reports also detail how much money is wagered on the slot machines under each denomination and how much was retained by the casino as revenue.

The annual reports provide a hold percentage based off those tallies. Hold percentages for individual slot machines are not publicly available.

Typically, a good hold rate from a gambler's perspective is around 6 percent. A 15 percent hold rate is relatively bad.

The new video slots at Illinois casinos have averaged hold rates over 12 months of about 10 percent to 15 percent, according to state records.