The old fashioned, mechanical slot machine had a series of vertical reels or wheels mounted inside, oriented from front to back. The minimum number of reels was three, and the upper limit was five. Four was the common number of reels in a casino slot machine. The reels spun independently of each other. The spin was caused by a player’s inserting a coin, which permitted him or her to pull a handle at the side of the machine. The slot machine also provided a small window over each wheel through which a player could see the wheels spin. When the wheels stopped, the player could then see the picture or drawing displayed by each wheel through the window. Pictures on the wheels were bananas, apples, cherries (all originally flavors of chewing gum), roses, bells, “bars,” dollar signs, “Lucky 7” and many others. Prizes were defined according to combinations of the pictures displayed, and if the spin was a winner, the correct number of coins would spew out of the machine at the bottom. Players would use large tin buckets to collect their coins.

Precursors of the modern slot machines first came on the scene in the 1890’s and by the early 1900’s they were found in bars all over the country. They were known as “slot machines” in the United States and “fruit machines” in the United Kingdom. Since the advent of electronic gaming machines, the mechanical reels have disappeared entirely. In 1964, Bally developed an electro-mechanical slot machine called “Money Honey.” Electro-mechanical machines are still in evidence in a few casinos. Slot machines have almost entirely switched over to video game technologies, permitting many different kinds of games with lots of complex variations. The basic concept remains the same: that of inserting coins and taking a “spin” for a prize. With the advent of semiconductors, new forms of machine gambling were invented, including video poker machines, bingo electronic game devices and Keno machines. These are discussed in other sections.

Some machines are “low level” machines or “slant tops” which means that there will be a chair or a stool nearby so they can be played while sitting. “Upright” or “Standup” machines are designed to be played from a standing position.

Some machines are said to be “progressives” in that there is a jackpot associated with the machine or with a bank (called a “carrousel”) of interconnected machines in an alcove in the casino. As people play the machines, the progressive jackpot grows, until someone wins it. Many people play the progressives when the jackpot is high on the mistaken premise that the machine is “due” for a win. Each bet is independent of each other bet, so a machine is just as likely to give a jackpot on the first spin after a jackpot as on the 1,000,000th spin after a jackpot. The only reason to play a progressive when the jackpot is very high relates to reducing as much as possible the house’s edge against the player. Unlike progressive jackpots on video poker machines, progressive jackpots on slot machines are not susceptible to the calculation of a “break even point,” meaning that point at which the size of the jackpot justifies a bet because the expectation is no longer negative. As it is not possible to figure out the probabilities of winning with a given machine, the breakeven point can not be calculated. However, it is always true that as the jackpot rises, the expectation of the player turns increasingly more towards positive territory.

In Las Vegas, more casino guests prefer slot machines to any other form of gambling. Approximately 70% of the casino’s income is derived from slot machines. This may help explain why, upon entering a casino, the player first sees the arrays of slot machines before passing to the table games.

In Nevada, it is required for video slot machines to have a minimum average payout of 75 percent.