Online Slots and their mechanical counterparts are by far the most popular gambling games in the world. Filling the floors of land-based establishments and dominating the lobbies of online casinos, slots have come a long way since their humble 19th century beginnings.

Basic Rules of Slot Machines

Basic mechanical slot machines require the insertion of a coin(s) which unlocks a side-handle. When this is pulled causes a set of decorated reels to spin, which eventually stop in sequence. If they line up in a winning combination, a pay-out is triggered. While this may seem relatively straightforward, the creation of such a mechanism is rather complicated.

History of Slots

Slot machines can be traced back to counter-top units of the late 19th century. Known as ‘trade stimulators’ these basic contraptions usually sat next to a cash register and dispensed fruit-flavoured confectionery (hence the cherry and melon symbols still used today).

The most famous of these trade simulators was the Guessing Bank, in which a coin was inserted, causing a dial to spin which then landed on a random number. A player that guessed the correct number before inserting the coin won a prize. Similar systems were devised by Chicago and New York Based companies in the 1890s. But instead of digits, the reels displayed playing cards.

The first slot machine that could offer automatic payouts was called the 3 for 1. Invented by John Lighton, it comprised a relatively simple mechanism featuring two coin channels. One led to the machine’s cash box, the other was fitted with a lever. When this lever was tripped, another two coins were released. Although fairly primitive, the 3 for 1 paved the way for automated payouts and a series of famous slot machines called the Bell variants.

The Bell Variants

The Card Bell and Liberty Bell were the first slot machines to offer automated payments. Invented by Charles Fey, the machines featured three reels instead of five. This reduction helped reduce the endless winning combinations making it easier for the machines to read a win. The Liberty Bell was also the first machine to include fruit symbols instead of playing cards or digits.

The popularity of the Liberty Bell led to mass imitations which continued well into the 1960s. However in 1963, the first electromechanical slot machine was introduced by Bally Entertainment. Known as the Money Honey, the machine featured a bottomless hopper which allowed for larger pay-outs as well as buttons instead of a lever. It became massively popular and led to the release of more sophisticated versions.

70s Electronica

By 1970, slot machines became fully electronic as manufacturers started to use solid-state circuitry. These eventually incorporated microprocessors to control internal mechanisms. But it was the introduction of a small computer called the random number generator which eventually did away with mechanically-triggered reels.


Using pre-programmed formulas, the RNG offered a more sophisticated, dependable method for ensuring accurate RTPs. It also enabled manufacturers to include video screens and other interfaces to display reels.

The Fortune Coin Video Slot was the first to use these new features. It comprises a 19-inch video screen and made full use of RNGs and logic boards to control all of its internal functions. After a series of modifications it was eventually approved by the Nevada State Gaming Commission. Thus, the video slot era had begun which takes us right up to the present day.

Online Slot Game Variants

Gradual improvements in computer processing power and internet technologies have led to a huge proliferation of online casino slots. In the early years of online UK casinos, games had to be downloaded for play. Now, gamers can play online slots within their web browsers. There are generally four types of online slot games.

  • Classic Slots – based on the classic three-reel configuration with minimal pay-lines
  • Video Slots – feature-laden games with at least five reels, extra symbols, up to 100 pay-lines and extra bonus games
  • Progressive Slots – linked online slots offering large, pooled prizes taken from a shared pot of player prizes
  • Mobile Slots – slot games specifically developed for use on smart-phone/mobile devices, often featuring simplified game-play

Slot Machine/Online Slot Terminology

3-Reel/5-Reel – number of reels in a slot game

Auto-Spin – a function which enables player to spin game reels automatically

Bet – amount of cash wagered on a spin

Bonus Round – additional game which sometimes offers instant cash wins.

Branded Slots – famous TV/Movie tie-ins used for slot games.

Cascading Reels – feature in which winning reel combinations are replaced with symbols above

Classic – online slots based on traditional mechanical slot machines

Drops – a feature in which symbols fall into place on the game reels

Feature – the biggest bonus round offered by a slot game

Free-to-Play – slot games which can be played without staking money

Free Spins – free games that are activated by a bonus round

Fruit Machine – UK term for land-based slot machines

Gamble – featured which allows players to double their winnings on a single spin

Jackpot – biggest possible pay-out

Maximum Bet – biggest possible stake

Multi-Line – game featuring more than one pay-line

Nudge – bonus feature which allows players to move reels up or down

One-Armed Bandit – classic slot machine where the reels are spun by a side-lever

Online Slot – web-based slot game

Payline – a line of symbols that pay out a prize

Payout – money awarded by a slot for landing a winning combination

Pay Table – list of game payouts and rules

Penny Slots – small stake slot games

RNG – program which controls reel spins

Reels – vertical columns decorated with symbols

RTP – return to player, usually displayed in percentages

Scatter – symbol which pays out in combinations wherever it lands

Skill Bonus – bonus game requiring ‘skill’

Spin – moves the slot reels during a game

Stacked Symbols – identical symbols one on top of the other which usually covers the entire reel

Stacked Wilds – wild symbols one on top of the other

Symbols – icons which decorate slot reels

Wild – symbol that substitutes others to create winning combinations

Wild Multiplier – additional wild bonus