Slot Tutorials

Slot Vocabulary: ‘Reels First’ Approach to Spin Outcomes

Written by Joshua

When it comes to Vegas-style slot machines, the traditional way of designing them is as follows:

  • The RNG is polled to get the determining values for the spin
  • The outcome for each reel is looked up based on what the RNG provides
  • The outcome is displayed on the screen to the player
  • The game evaluates the outcome to determine if there’s a win or not

This technique is called a “Reels First” approach, and is the traditional approach to the design of Vegas-style slot machines in determining a spin’s outcome, and whose process kicks off the moment a player hits the spin button.

In the case of this approach, an RNG outcome is actually determined for each reel, and the outcome for each reel then revealed. This ensures a completely random outcome based on the design of the reel set and the weighted distribution of reel stops, as applicable.

So when a Vegas-style slot machine’s RNG is discussed, it’s not, in this format, a single RNG call that determines the value of all five reels, but instead, each reel has its own RNG call that is done to determine where each reel comes to a stop, regardless of whether it’s a mechanical or virtual reel set.

Then, once the outcome of all five reels is determined based on those calls, the game can evaluate the symbols on the reels, determine what, if any, payouts there should be, and then pay the player.

This all happens virtually instantaneously thanks to the speed of computers to handle these operations, but it does play out accordingly. Most slot machines on a casino floor will operate this way, but there’s another way to handle the outcome of a slot spin, and I’ll define and explain that in next week’s post.

About the author


My name is Joshua, and I’m a slot enthusiast who works in tech as a marketer by day, and dabbles in casinos periodically during off-times. Know Your Slots will reflect my interests in understanding the various ways you can play slots, travel, casino promotions and how you can get the most out of your casino visits.

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