It will be mere waste of time if we try to categorize slot machines. Since each casino tries to attract players by the wide selection of one arm bandits, there are hundreds of different types and variations - single line, three line, 3 or 5 reel, progressives, bonus games and many others.
All we can do is to list some parameters that can be used while describing a slot machine, including online slots.
Number of Reels
Nowadays you can easily find slots with five reels. These modern machines usually have more than one payline (see below).
Number of Rows
There are one, two or more horizontal rows across vertical reels displayed by the slot machine.
The additional rows, one each above and below the payout window allow players to "see" how close they had come to winning.
Number of Pay Lines
A slot machine can have one or more (3, 5, 9 or even 15) pay lines on its reels where symbols must line up on to be a winning combination. Usually it's the center row of symbols across vertical reels.
Classic slot machines have only 1 payline that goes through the middle of each 3 reels. You can wager more than one coin per spin - but the amount will apply to wins on the same payline and your chances of winning will be the same.
If a machine has more than one payline, a player can activate additional ones by increasing his/her bet, but have the option of not playing all available lines.
For example, if you play on a 3 line machine, you should insert one coin to play on the center line, two coins for the center line and the line above it or 3 coins for all 3 lines . So, if you play on the center line with one coin and the winning combination comes up on the bottom line you will not win.
Slot machines with 5 reels can have from 5 to 15 paylines with the diagonal matches.
Denomination of one coin
Slot machines can be found in many denominations from 5 cents to $5. High roller machines can take denominations as high as $500.
Number of coins that can be played
Usually it's a three coins machine, but machines with a two coin maximum bet are also available.
Progressive Slots are linked in a bank of machines so that a certain percentage of every bet goes to the progressive jackpot pool. This jackpot pool belongs to a group of slots in a casino, or even in different casinos.
The more people playing on the linked machines, the faster the jackpot increases. Finally one lucky player will scoop the jackpot he/she hits the highest winning symbol combination. To hit the jackpot the maximum coins must be played.
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Multiplier slot machines
A multiplier slot machine doesn't have a jackpot, but the payback for additional coins is a multiple of the payback for a single coin.
Double slot machines
Some slots have an additional wheel that can be played after a win. So a player has a 50/50 chance of doubling (or losing) his/her winnings.
Nudge machines have a light mounted on them that comes on occasionally. When it on, the player may move (nudge) a wheel forward or backward one location to get a certain combination of symbols on the payline, so a losing bet can become a winner. The house edge on these machines is usually the same as for regular slots.
The special prizes is waiting for you if you get a special symbol combination or collected certain symbols. If you do, you will enter a bonus round. Usually a player have to bet maximum coins to qualify for the bonus round.
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These machines have a special wheel mounted on top. When the winning symbols appears on the slot machine this wheel spins and the player has a chance at winning the wheel-of fortune jackpot.