All About Slots
The first 'slot machine' came about in 1891 in New York, and was a mechanical 5 reel game based on 5 card poker. It was used mostly in bars and had no payout mechanism, the player would basically have an employee verify their wins manually after a winning spin. A few years later, the game was simplified by another inventor to have just 3 reels with 5 symbols on each, which made it possible with the limited technology at the time to introduce automatic payouts for winning spins. This game was called Liberty Bell, and was the design that was copied and modified over decades to come.
The first electric/mechanical hybrid machine was introduced in the early 60's, the combination of which allowed for better automated payouts - and many physical slots still utilize a combination of actual 'reels' and electronics to generate random outcomes to this day. Pure video slots didn't arrive until more than a decade later, no longer requiring traditional mechanical parts. Over time these became more complex, adding modern features such as bonus rounds, free spins, various animations and other things that are now commonplace.
Slot machines don't really warrant much of how a 'how to play' section, frankly there's not much to it. Put your money in the machine, press spin and cross your fingers that you spin something good - that's the game in a nutshell. Assuming you can change your bet (as you can with most online slots) and choose the number of lines to bet on there's a little more to it, but not much.
Choosing Your Bet:
Some machines are 'fixed' (usually only progressive jackpot games if playing online) and don't allow the player to change their betting stakes, you must play for $x amount each time you spin. Most non-progressive games however will allow you to choose the amount you wish to play for, whether it's 20 cents a spin or $100 (different games have different betting ranges, and different casinos may have different betting options on the same games.)
The interface for changing your bet will vary depending on the game being played, make sure you know what the bet amount represents - the total bet, or bet per line. There's a huge difference between playing 25 cents a spin and 25 cents per line each spin on a 20 line game. Another quick note, when playing online some games will remember your last session and retain your bet settings, some don't. Before you spin make sure your total bet is as intended.
Choosing Your Lines:
Some slot games will allow you to choose the number of lines you'd like to play, for example a slot may have 10 pay lines in total, but allows the player to choose anywhere from 1 to all 10 lines per spin. Playing fewer lines naturally costs less per spin, but also naturally reduces your chances of winning any given spin. Games where this is an option will always have a bet per line setup rather than a bet per spin, so again make sure your total bet is something you're comfortable with. Generally speaking, it's better to play all lines at a lower bet per line than fewer lines at higher amounts.
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Types of Slots
Once upon a time this was a much simpler differentiation, there were "traditional" slots - 3 reel, mechanical games and "video" slots - electronic, usually 5 reel games with no actual moving parts. In brick and mortar casinos the line has since been blurred with hybrid machines (part mechanical, part electronic - some even have both physical wheels and video screens,) and online it's hard to call anything a 'traditional' slot even if that's what the game is modeled on - since it's ultimately played on a screen and not mechanical at all.
So rather than try and over-simplify 'types' of machines, instead I'll go over different options and features today's slots may have. These days a 3 reel slot can have bonus rounds, and a 5 reel slot can have no features at all - it just depends on the game.
One of the first things you'll notice is how many reels a slot machine has. 5 is the most common in today's gambling scene, but there's still a decent number of 3 reel slots around. Some of today's games have many more than 5, and some games make it hard to even call them 'reels' in the traditional sense.
A pay line is a path across a slot machine reels that pays the player if particular symbols appear on that 'line'. The earliest of games had just one, running horizontally across the middle of 3 reels. Today even 3 reel slots generally have five, 3 horizontal pay lines in the middle, top and bottom, and two diagonal lines from top to bottom and bottom to top. Different slot machines have different numbers of pay lines - video slots can have upwards of 100, and some games will pay on any left to right combination of symbols even if they're not 'connected.' In fact some games don't even have pay lines, instead paying on groups of symbols that intersect with one another (i.e. 5 touching symbols anywhere on the 'reels.')
Plenty of video slots have 'wild' symbols which act as whatever is needed to complete a winning line. Sometimes scatters (see below) may act as wilds, usually these are distinct symbols though.
Certain slot machines have what are called 'scatter' symbols, which generally pay when 3 or more appear during a spin. What makes these different than regular line wins is their position is irrelevant (they can be 'scattered' anywhere across the reels.) In addition to a payout, these are also often used to initiate free spins or an alternative bonus round.
These can be initiated in various ways, by particular 'free spin' symbols coming up (usually 3, but sometimes more or less,) by scatter symbols coming up (again usually 3 or more) or sometimes just randomly on any spin. As you'd expect, they award the play a certain number of free spins once hit. These free spins may (or may not) have additional perks, such as a multiplier, extra wild symbols or something else to differentiate it from the base game and spice things up. On most games free spins can also be 're-triggered' by hitting the same triggers that started it, on some additional free spins may be awarded each time a certain symbol appears (i.e. +1 free spin if a free spin symbol appears during free spins,) and on some there's even secondary free spins with additional perks that can be triggered when certain things happen on the reels.
While free spins certainly qualify as a bonus round, some games have bonuses that are very distinct from the base game as well - not involving spins at all. A pirate-themed game might have a bonus in which you pick one of ten chests for a random win for example. Another game may have a prize wheel to spin like Wheel of Fortune games. There's infinite possibilities for bonus rounds, and some are quite creative and fun.
Some slots offer 'progressive' jackpots, which are fed into each time someone plays that particular game. These jackpots are normally 'seeded' with a certain amount of money, and every spin on the game increases the jackpot by a certain amount (whether it's a penny per spin or 1% of the bet total) until the jackpot is won. Win conditions for said jackpot vary, it can be a near impossible reel combination, or sometimes a random opportunity to win it that's more likely to appear with higher bets (both in coins and overall bet.) These can get massive, and have paid out over $20,000,000 on more than one occasion from online slots. Many of these games have a fixed bet, and if not may have a minimum bet requirement to be eligible for the jackpot.
Some games will allow the player to respin a particular reel at additional cost if they almost hit something but didn't, basically a second chance. (I don't care for these games much personally for various reasons.)
These types of slots come in different varieties, but the idea is that after a winning combination, the win is paid and the winning symbols are removed from the board to make way for new ones (on the same 'spin' at no additional cost.) Most often the symbols are removed and those above them 'fall' down to occupy there space while new symbols drop in from the top, but they may be replaced without the shifting of reel symbols as well. These are good examples of games that don't have what you'd consider traditional 'reels.'
Slots are completely random - there's nothing the player can do to increase their odds after they spin. Before they do however, there's a couple of things to keep in mind.
It can be hard to find out for certain games, but slot machines have what's called an RTP (Return To Player) average. This is the amount over time that a game will pay back to it's players. For example, a game with a 90% RTP would pay back $90 of every $100 wagered over time. This is not a short term process, it's entirely possible to lose $100 without even a single win - but programmatically, the odds have been designed to achieve that percentage. The more the game is played, the closer the machine will get to that average.
In Vegas casinos, most games have anywhere between an 88% to 93% RTP on their games as an average. Online slots usually do a few points better than that in with 94% - 95% range, with some outliers reportedly as high as 99%. While it's fun to play a variety of games, if you find one you particularly like see if you can find that RTP before you become too smitten with it, some of the most fun games may offer some of the worst payback.
Like video poker, some slot machines will offer bonus payouts or jackpots only when the player has made a certain minimum bet. Progressive jackpots will typically only be triggered on a max bet for example. If that bet amount is more than you're willing to bet, you're probably better off looking for a different machine to play - as the machines overall payback will have these bonuses and/or jackpots taken into consideration (your RTP will be much lower otherwise.)