Almost everyone will have seen symbols used in slot machines at some stage in their life, with these game symbols being used throughout the world for decades on various real-world slot machines, including slots online. Even those who have never actually played themselves in order to see the slot machine symbols, they're frequently used on TV to advertise the classic slots game that so many people know and love.
It's matching a combination of these slot machine symbols that determines what, if anything, a player has won. In the earliest days of the invention, the winnings had to be paid out manually, but in 1895 the symbols many will recognize today were introduced as part of upgrades to the game that made payouts automatic. Three suits of cards were left on the machines (diamonds, spades, and hearts), which now had only three reels, and the lucky horseshoe and liberty bell were also added to the mix. It's hard to know when the lucky 7 became a prominent feature in the classic slots game, but the number has always been considered fortunate by gambling punters.