A reel is one of the vertical columns of symbols on a slot machine or video slot. On mechanical machines, each reel was a physical drum carrying a printed strip of symbols; on video slots, it is a software reel whose stopping position is chosen by the random number generator. When a spin ends, the symbols showing on each reel are read across the paylines or ways to decide whether the spin wins.
Classic slots use three reels, while five reels is the standard layout for modern video slots. Developers also build six- and seven-reel games, grids with reels of different heights and titles with two sets of reels side by side. For operators, reel layout is one of the first filters players use, so lobby tags such as classic, five-reel or Megaways help them find the games they like.