Originally, casinos installed slot machines as a
diversion for casual gamers. Unlike traditional table
games (such as blackjack or craps), slot machines don't
require any gambling knowledge, and anyone can get in the game
with a very small bet.
This idea proved to be a monstrous success -- slot machines
eventually moved off the sidelines to become the most popular
and the most profitable game in town, bringing in more than 60
percent of the annual gaming profits in the United States.
The technology of slot machines has also changed a lot over
the years. The classic mechanical designs have been almost
completely replaced by computer-controlled machines. But the
game has remained the same. The player pulls a handle to
rotate a series of reels (typically three) that have
pictures printed on them. Winning or losing is determined by
which pictures line up with the pay line, a line in the
middle of a viewing window. If each reel shows the same
winning picture along the pay line, you win (certain single
images are sometimes winners as well). The amount you win --
the payout -- depends on which pictures land along the
In this edition of HowStuffWorks,
we'll find out what sets the reels in motion in modern slot
machines as well as in the old mechanical models. We'll also
see what determines the odds of winning on a slot machine and
look at some popular variations on the traditional game.
The One-Armed Bandit The classic slot
machine design works on an elaborate configuration of gears and
levers. The central element is a metal shaft, which supports
the reels. This shaft is connected to a handle mechanism that
gets things moving. A braking system brings the spinning reels
to a stop, and sensors communicate the position of the reels
to the payout system. A coin detector initially registers that
a coin has been inserted and unlocks a brake so the handle can
There are any number of ways to arrange these elements, and
manufacturers have tried dozens of approaches over the years.
The diagram below shows one representative design.
Click on the three "click here" areas to see the
different parts of the mechanism animated. Then read the
description below for details
This design includes three reels mounted on a central
shaft. The central shaft also supports three notched
discs, which are connected to the three reels. A second
shaft below the central shaft supports a kicker, a
piece of metal comprising three paddles. The kicker paddles
are lined up so they can push against the notches on the three
discs. The second shaft also supports a series of connected
stoppers, teeth that lock into the notches on the
The kicker and the stoppers are both connected to springs,
which hold them in a standby position. The kicker is held in
place behind the discs, while the stoppers are held up against
the discs, locking them into place.
Here's what happens when a player pulls the handle:
The handle rotates a hook mechanism, which grabs hold of
the kicker, pulling it forward (toward the player).
A catch on the opposite end of the kicker grabs a
control cam piece and pivots it forward. This rotates
a series of gears connected to the control cam. A
spring pulls the control cam back to its original position,
but the gear assembly slows it down considerably -- the
gears act as a mechanical delay.
When the control cam is pivoted forward, it releases a
spring-mounted cam plate extending across the back of
The control cam also pulls the stoppers away from the
notched discs. As the kicker keeps moving, it pushes the
stoppers against several catches on the cam plate. These
hold the stoppers in place, so the discs and reels can
As the handle continues to move the kicker, the kicker
paddles push the discs forward briefly. When the handle is
pulled all the way back and the kicker has passed the discs,
the bottom of the hook mechanism moves up against a slanted
surface. The slant pivots the hook forward, which causes it
to release the kicker.
The kicker spring jerks the kicker backward at a good
speed. The kicker paddles hit the notches on the discs,
spinning the reels rapidly.
While all of this is happening, the control cam is
slowly returning to its original position. When it does
return, it pushes the cam plate back, which releases the
stoppers. The different catches holding onto the different
stoppers are positioned so that the cam plate will release
the stoppers one at a time. Each stopper springs forward and
locks into a notch, holding the reel in position.
From the player's point of view, here's how it looks. The
player pulls the handle. There is a clunk, and the three reels
start spinning. Then the three reels stop abruptly one at a
time, followed by the payout (if necessary). The "stopping one
at a time" part builds suspense. If the first reel stops on
the jackpot symbol, then you have to wait for the next reel to
stop to see if it is a jackpot, and then finally the third. If
all three display the right symbol, the player wins.
Conventional mechanical slot machines eventually gave rise
to electrical machines that worked on similar
principles. In an electrical machine, the reels are spun by motors and
the stoppers are generally activated by solenoids,
but the game basically plays out the same way. Electrical
machines have more sophisticated money-handling systems, like
those you might find in a vending machine, and flashier light
and sound displays.
In both types of systems, once the reels have come to a
stop, the slot machine needs to read whether the player has
won or lost. In the next section, we'll examine some systems
for making this determination.
Payout There are dozens of different payout
systems used in slot machines. In one of the simplest designs,
a jackpot is detected by measuring the depth of notches in the
discs that drive the reels. For simplicity's sake, we'll look
at this sort of payout system in a bare-bones slot machine.
The machine only accepts one kind of coin, and there is only
one winning combination of images.
When you put a coin in this machine, it falls into a
transparent case. The bottom of the case is a movable
shutter that is connected to a metal linkage, as you
can see in the diagram. Normally, the linkage holds the
shutter closed. But when the machine hits the jackpot, the
third stopper shifts the linkage up, opening the shutter so
the coins fall out of the machine.
Each of the three discs has notches for each stop position
of the reel. The notch for the jackpot stop is deeper
than the other stops. Consequently, when the first reel lands
on the jackpot stop, the first stopper moves farther to the
left than it would for any other stopper. If the second reel
stops on the jackpot as well, the second stopper also moves
farther left. Same goes for the third reel and stopper.
But if only the second reel stops on the jackpot, the
second stopper will not move all the way into the notch. The
first stopper has a catch that keeps the second stopper from
moving past it. The second stopper, in turn, has a catch that
holds the third stopper back. For the third stopper to lock
all the way into the jackpot notch, then, the first and second
reels would have to have landed on the jackpot image. When
this happens, the shutter opens to dump all of the coins that
have been played since the last jackpot.
Typically, slot machines will have more elaborate versions
of this design in order to pay out partially on certain
combinations of images and pay out completely on the jackpot
In another popular system used in some electrical machines,
the discs have a series of metal contacts attached to
them. When the reels stop, one of the contacts engages a
stationary contact wired to a circuit board. In this way,
every stop on each reel will close a different switch in the
electrical system. Certain combinations of closed switches
(jackpot winners) will configure the machine's electrical
circuit to operate the payout mechanism.
A more advanced system uses photoelectric
cells (also known as photo diodes), devices that
generate a current when exposed to light, to
detect the position. In this system, a series of holes are
drilled through the rotating discs, all around their outer
edges. The photo diode is positioned on one side of the disc,
and a light source is positioned on the other side. As the
disc turns, the light shines through the holes onto the photo
diode. The pattern of holes in the disc causes the photo diode
to generate a similar pattern of pulses of electricity. Based
on this pattern, an electronic circuit can determine the
position of the reel.
In the past 15 years, electric machines and fully
mechanical machines have both been eclipsed by computerized
machines. In the next section, we'll see how these modern
Modern Machines Most modern slot machines
are designed to look and feel like the old mechanical models,
but they work on a complete different principle. The outcome
of each pull is actually controlled by a central computer
inside the machine, not by the motion of the reels.
The computer uses step motors to turn each reel and
stop it at the predetermined point. Step motors are driven by
short digital pulses of electricity controlled by the
computer, rather than the fluctuating electrical current that
drives an ordinary electric
motor. These pulses move the motor a set increment, or
step, with great precision (see this
page to find out more about step motors).
But even though the computer tells the reels where to stop,
the games are not pre-programmed to pay out at a certain time.
number generator at the heart of the computer ensures that
each pull has an equal shot at hitting the jackpot. (Click
here to find out how random number generators work.)
Whenever the slot machine is turned on, the random number
generator is spitting out whole numbers (typically between 1
and several billion) hundreds of times a second. The instant
you pull the arm back (or press the button), the computer
records the next few numbers from the random number generator.
Then it feeds these numbers through a simple program to
determine where the reels should stop.
Here's how the complete process plays out in a typical
You pull the handle, and the computer records the next
three numbers from the random number generator. The first
number is used to determine the position of the first reel,
the second number is used for the second reel and the third
number is used for the third reel. For this example, let's
say the first number is 123,456,789.
To determine the position of the first reel, the
computer divides the first random number by a set value.
Typically, slot machines divide by 32, 64,128, 256 or 512.
In this example, we'll say the computer divides by 64.
When the computer divides the random number by the set
value, it records the remainder of the quotient. In our
example, it finds that 64 goes into 123,456,789 a total of
1,929,012 times with a remainder of 21.
Obviously, the remainder can't be more than 64 or less
than 0, so there are only 64 possible end results of this
calculation. The 64 possible values act as stops on a large
Each of the 64 stops on the virtual reel corresponds to
one of the 22 stops on the actual reel. The computer
consults a table that tells it how far to move the actual
reel for a particular value on the virtual reel. Since there
are far more virtual stops than actual stops, some of the
actual stops will be linked to more than one virtual stop.
Computer systems have made slot machines a lot more
adaptable. For example, players can now bet money straight
from a credit account, rather than dropping coins in for every
pull. Players can also keep track of their wins and losses
more easily, as can the casinos. The operation is also simpler
in modern machines -- if they want to, players can simply
press a button to play a game, rather than pull the handle.
One of the main advantages of the computer system for
machine manufacturers is that they can easily configure how
often the machine pays out (how loose or tight
it is). In the next section, we'll see how the computer
program can be configured to change the slot machine's odds of
hitting the jackpot.
What are the Odds? In a modern slot machine,
the odds of hitting a particular symbol or combination of
symbols depends on how the virtual reel is set up. As
we saw in the last section, each stop on the actual reel may
correspond to more than one stop on the virtual reel. Simply
put, the odds of hitting a particular image on the actual reel
depend on how many virtual stops correspond to the actual
In a typical weighted slot machine, the top jackpot
stop (the one with the highest-paying jackpot image) for each
reel corresponds to only one virtual stop. This means that the
chance of hitting the jackpot image on one reel is 1 in 64. If
all of the reels are set up the same way, the chances of
hitting the jackpot image on all three reels is 1 in 643, or 262,144. For machines with
a bigger jackpot, the virtual reel may have many more stops.
This decreases the odds of winning that jackpot considerably.
The losing blank stops above and below the jackpot
image may correspond to more virtual stops than other images.
Consequently, a player is most likely to hit the blank stops
right next to the winning stop. This creates the impression
that they "just missed" the jackpot, which encourages them to
keep gambling, even though the proximity of the actual stops
A machine's program is carefully designed and tested to
achieve a certain payback percentage. The payback
percentage is the percentage of the money that is put in that
is eventually paid out to the player. With a payback
percentage of 90, for example, the casino would take about 10
percent of all money put into the slot machine and give away
the other 90 percent. With any payback percentage under a 100
(and they're all under 100), the casino wins over time.
In most gambling jurisdictions, the law requires that
payback percentages be above a certain level (usually
somewhere around 75 percent). The payback percentage in most
casino machines is much higher than the minimum -- often in
the 90- to 97-percent range. Casinos don't want their machines
to be a lot tighter than their competitors' machines or the
players will take their business elsewhere.
The odds for a particular slot machine are built into the
program on the machine's computer chip. In most cases, the
casino cannot change the odds on a machine without replacing
this chip. Despite popular opinion, there is no way for the
casino to instantly "tighten up" a machine.
Machines don't loosen up on their own either. That is, they
aren't more likely to pay the longer you play. Since the
computer always pulls up new random numbers, you have exactly
the same chance of hitting the jackpot every single time you
pull the handle. The idea that a machine can be "ready to pay"
is all in the player's head, at least in the standard system.
In casinos today, gamblers will find a wide variety of
slot-machine designs. In the next section, we'll look at some
variations on the standard game.
Variations When you hit the slot machines in
a casino, you'll have dozens of gaming options. Machines come
with varying numbers of reels, for example, and many
have multiple pay lines.
Most machines with multiple pay lines let players choose
how many lines to play. For the minimum bet, only the single
line running straight across the reels counts. If the player
puts more money in, he or she can play the additional
horizontal lines above and below the main pay line or the
diagonal lines running across the reels.
For machines with multiple bet options, whether they have
multiple pay lines or not, players will usually be eligible
for the maximum jackpot only when they make the maximum bet.
For this reason, gambling experts suggest that players always
bet the maximum.
There are several different payout schemes in modern slot
machines. A standard flat top or straight slot
machine has a set payout amount that never changes. The
jackpot payout in a progressive machine, on the other
hand, steadily increases as players put more money into it,
until somebody wins it all and the jackpot is reset to a
starting value. In one common progressive setup, multiple
machines are linked together in one computer system. The money
put into each machine contributes to the central jackpot. In
some giant progressive games, machines are linked up from
different casinos all across a city or even a state.
Some slot-machine variations are simply aesthetic. Video
slots operate the same way as regular machines, but they
have a video image rather than actual rotating reels. When
these games first came out, players were very distrustful of
them; without the spinning reels, it seemed like the games
were rigged. Even though the reels and handles in modern
machines are completely irrelevant to the outcome of the game,
manufacturers usually include them just to give players the
illusion of control.
These are only a few of today's popular slot variations.
Game manufacturers continue to develop new sorts of machines
with interesting twists on the classic game. A lot of these
variations are built around particular themes. There
are now slot games based on television
shows, poker, craps and horse racing, just to name a few.
To learn more about modern slot machines, including
strategies to increase your chances of winning, check out the
links on the next page.