Virtual Reels and Reel Strips Virtual Reels & Reel Strips
The idea that the reels you see on screen differ from the reels that set the real probabilities. How many of each symbol are placed on the reel strip determines each symbol's chance of appearing and the RTP.
| English | Virtual Reels & Reel Strips |
|---|---|
| Classification | Math & Probability |
| English | Virtual Reels / Reel Strips |
| Category | Math · Probability |
| Patent | Inge Telnaes (US patent, 1984) |
| Key point | The number of visible positions differs from the number of real probability positions |
| Determining factor | Symbol makeup and count on the reel strip |
| Related | RNG, hit frequency, near miss |
Overview
The reels you see on a slot screen usually show 3 to 4 positions per reel. But what actually decides the result is a much longer symbol list behind the screen, called the reel strip. An invisible reel with far more positions than a mechanical one is called a virtual reel.
The probability of each symbol appearing, the hit frequency, and the RTP are all determined by which symbols, and how many of each, are placed on this reel strip.
Limits of mechanical reels
Old mechanical slot machines had about 20 to 22 symbol positions on each physical drum reel. With 3 reels and 22 positions, there are only 22×22×22 = 10,648 possible combinations, so a big jackpot would hit too often for a casino to afford.
The arrival of virtual reels
A 1984 US patent granted to Norwegian engineer Inge Telnaes solved this problem. A random number generator (RNG) picks a number between, for example, 1 and 64, and maps that number to one of the 22 positions on the physical reel. If you assign just 1 of the 64 virtual positions to the jackpot symbol and many positions to blanks, you can lower the jackpot probability as far as you like.
| Category | Physical mechanical reel | Virtual reels |
|---|---|---|
| Positions per reel | About 22 positions | 64, 128 positions, or any number |
| Number of 3-reel combinations | 10,648 | 262,144 (based on 64 positions) |
| Adjusting jackpot probability | Difficult | Freely, by assigning positions |
Probability calculation example
Say the reel strip has 50 positions, with 2 positions for the 7 symbol and 10 for cherries.
| Situation | Calculation | Probability |
|---|---|---|
| Chance of a 7 stopping on the middle row of one reel | 2 ÷ 50 | 4% |
| 7 on all 3 reels | (2/50)³ | 0.0064% (about 1 in 15,625) |
| Cherry on all 3 reels | (10/50)³ | 0.8% (1 in 125) |
Multiply the probability of each combination by its payout and add them all up, and you get the RTP. Providers adjust the symbol counts on the reel strips little by little to match the target RTP and volatility. Most games with several RTP versions are also made by changing the reel strips or probability tables (RTP versions).
Different strips per reel
Reel strips can differ from reel to reel. For example, if reels 1 and 2 contain many high-payout symbols and reel 3 contains few, you’ll often see them on the first two reels and then miss on the last one. That’s why the feeling of “almost got it” comes up so often.
The near-miss controversy
If you place many blanks directly above and below the jackpot symbol, you often get scenes where the jackpot symbol stops right next to the winning line. This kind of “narrowly missed” scene is called a near miss. The actual probability stays the same, but players may feel they “almost won” and keep playing, so regulators in many countries restrict or ban designs that deliberately create near misses.
In online slots
Online slots follow the same basic principle. The RNG picks numbers, those numbers are converted into reel strip positions, and the symbols are shown on screen. However, games where the reel height changes, like Megaways, or where symbols fall, like tumbling games, use several stages of probability tables in addition to the reel strips to decide the result. This probability structure is verified by testing labs, and is published to players as a summary in the form of RTP and volatility.