Invented in San Francisco, California at the very end of the nineteenth century, slot machines are the only fully mechanized form of gambling we can find in casinos today.

Games such as video poker etcetera are also technically slot machines, but what we are really talking about here are games where the player spins a reel to try to get a line of matching symbols.

Although the overall odds for the player are usually not the best among all the games offered, slot machines remain extremely popular with gamers because it is thrillingly possible to win very large jackpots from very small stakes, and also because the rules and methods of playing them are really incredibly simple.

Casinos also like these "one armed bandits" because they have relatively high profit margins and can be operated with very minimal staff involvement, so the overheads are low.

To play, you just pick a machine, insert your bet into the slot, press the start button or pull the start lever – the lever is more common on most machines because for some reason it is more satisfying to most people to pull a lever than it is just to push a button – and watch the symbols on the display spin until they come to rest.

Most machines usually have playing instructions printed on them and if in doubt, casino staffers are always friendly and ready to answer any question you might have.

A slot machine usually has four player control buttons and / or levers: spin, to play; cash out, to pay out all accumulated winnings if any and remaining play credits; bet maximum, to bet the maximum number of credits on a play; and bet one, to add a single credit unit to a bet before a play is made.

Many machines also have an auto play feature which allows a player to dispense with the necessity of having to keep pressing the spin button or pulling the lever, but most players prefer to play manually.

Before we go on, let us take a look at how slot machines work on the inside – literally in the case of real casinos, virtually in the case of online slot machines (the reputed popularity of online slots is surprising but absolutely true).

Many people imagine that when you pull the lever, some sort of random generator is triggered that determines by chance exactly where each wheel on the barrel is going to stop.

Nothing could be further from the truth, as there is no such thing as a machine that can generate a truly random number.

All computers are programmed with a "RND" generator but the generator already contains a sequence of pre-programmed numbers. So it is sadly true to say that the consecutive results of every future pull of the lever of every single slot machine in the world are already pre-determined!

Of course, that means that the number and size of all the winning plays are programmed into the machine, and the key to the program is what percentage of money paid into the machine as stakes are paid out as winnings.

The usual figure commissioned by most casinos is somewhere between ninety and ninety-seven percent paid out, giving a casino a profit of between ten and three dollars for every one hundred dollars put in the slot.

Most modern machines allow a range of bets. For example, a machine may let you bet anywhere from one dollar to five dollars, and if you win, the winnings you will receive are multiplied by whatever factor of five you inserted in the first place.

Slot machines accept bets ranging from a nickel to one hundred dollars. It is possible to pay with cash or to buy a computerized card that you can slip into the machine and debit as you play. When a player wins a relatively small amount, this is usually paid in cash.

A large win or a jackpot is dealt with by setting off a flashing siren above the machine to alert staff.

For the casino of course this also has the advantage of letting the other players see a happy winner getting their money! When any win occurs, the amount won is usually displayed in a special "win box" on the screen of the machine, and the relevant section in the payoff table is highlighted.

Every machine you find in either a real or virtual online casino will show how much it pays for a combination of particular symbols.

Usually the lowest payments start from two of a particular symbol showing from left to right, and then the biggest winnings come from getting all three reels in a row.

However, there are more and more modern slot machines that show a line of five symbol reels instead of the more traditional three, and many of these machines allow a player to choose how many symbol reels to spin: either three, four or five.

For some reason the cherry symbols often pay the lowest, and unsurprisingly, the jackpot symbols pay the jackpot.

Some machines pay off on diagonal as well horizontal lines but the player usually needs to pay extra for the chance of winning on the extra lines, up to a total of nine possible winning lines.

A player can be sure and double check exactly which horizontal and / or diagonal lines have been paid for before a play by seeing which red lines have come up on the display screen before pulling the lever.

A red line shows that this line has been paid for. Each reel usually has about twenty-two different symbols or blanks that may show up on a particular spin.

This means that a three reel slot machine has over ten thousand combinations that may show up on a particular spin and a five reel slot machine has over five million!

Of course, the symbols used are irrelevant in themselves. Machine manufacturers make many "themed" machines using all kinds of inventive novelties.

The classic style has cherries, bars, sevens, and jackpots. But enter a casino and you can find wild west themes, ghost themes with monster symbols and creepy music, even machines based upon popular television programs or films like "Titanic" which use different types of ships as symbols.

Symbols I have seen include rockets, planets, dogs, pyramids, knights, flowers, acorns and piggy banks! Some of these slot machines even play themed music. The varieties are literally endless. The last twenty years has seen the incorporation of video technology into the slot machine.

So far, so simple. Where it starts to get a bit more complicated is in how the jackpots are worked out when a casino offers a single "progressive" jackpot common to all the machines at any given time.

Here all the slot machines are hooked up to a computer that records the amount of money put in to all the slots and uses this number to build up a proportionate jackpot.

In a casino you will usually see the current total of this jackpot advertised prominently on LED screens at the entrances and also over each slot machine. There are even some slot machines hooked up to a jackpot connected to outside casinos.