In a slot game a reel can be defined as a unit that spins independently. In the original mechanical slots there used to be three physical reels that would spin independently and display these symbols in a line when they came to rest. In online slots there are no physical reels spinning, but the spin mechanism is simulated using a random number generator. But the concept of reels is still valid. Online slots have from three reels to nine reels. Each format has its own characteristics, which will be discussed in this article.

The earliest online slots were three reel slots developed to exactly replicate the mechanical slot machines. These had only one row of symbols on display and hence only one payline. In order to create more winning combinations online slots began to display three rows of symbols, with each row forming a horizontal payline. Then slots began to use the diagonal lines as paylines and hence three reel slots with four and five paylines were created. Flower Power from Cryptologic and Happy New Year from Microgaming are examples of three reel slots with five paylines. Ultimate Fighters from Playtech is a unique three reel slot game that has eight paylines. Along with the horizontal and diagonal lines it uses the three vertical lines as well. In some classifications another Playtech slot called Ugga Bugga is listed as a three reel and ten payline slot game. However this is different from the multi-line three reel slots described earlier because in this case there are actually 10 single paylines that spin independently.

In order to increase the number of paylines five reel slots were introduced. These had five reels aligned next to each other forming a matrix of five columns and three rows. Paylines were not restricted to straight lines and were allowed to zigzag across the reels. In this manner online gaming software developers were able to introduce five reel slot games with a large number of paylines. Microgaming has released two 5 reel slots with hundred paylines. They are Cashapillar and Summer Holiday. These slot games have four rows of symbols instead of the usual three. Theoretically a five reel slot game with three rows can have up to 243 Paylines. Microgaming has released one such slot called Burning Desire. Only it does not define it as a game with 243 paylines but with 243 ways to win. The slot calculates wins on left-to-right combinations regardless of their position on each reel. Almost all the online slot games on offer are either three reel or five reel slots.

Vegas Technology has released two online slot games with seven reels. As in the earlier cases the reels are aligned next to each other and three rows of symbols are displayed. Lucky 7s has seven paylines and Farming Futures has nine paylines. One of the most interesting formats of online slot games is the nine reel format. The nine reels are not aligned next to each other as before but in a three by three matrix. Each reel displays only one symbol and hence all the nine slots spin independently. Playtech has released a number of such games. One of its most popular progressive jackpots, Gold Rally, is in this format. Another Playtech slot in this format is Chinese Kitchen. The Cryptologic online slot Sweet Thing is also in this format. All games have eight paylines - three horizontal, three vertical and two diagonal.

There are a number of slots released by Cryptologic in its Casual Game series which are structured in a different way altogether. The concept of reels does not really apply. Cubis, and the recently released, Jenga are two such games.