With 100 stops on each of three reels, there are 1 million combinations. The change to virtual reels enabled programmers to make the reels to behave as if they had any number of stops. With bigger reels holding 20 symbols and 20 spaces, combinations increase to 40x40x40, or 64,000.

If blank spaces between lines are used as stops, the number of combinations increases to 20x20x20, or 8,000.

On early three-reel slots with 10 symbols per reel, there were 1,000 possible combinations (10 x 10 x 10).

The sheer number of possibilities makes the math that goes into slot machine odds more complicated than on table games. The number of winning combinations and the payoffs per winner work together to determine a game’s odds.

There also is an open field for game designers to assign how much each winning combination pays. There are thousands, and sometimes millions, of reel combinations. Slot machine odds work in a similar fashion to the roulette example, except there are many more possibilities on the slots.

How to win at slot machineswikipedia