Random encounters were incorporated into early role-playing video games and have been common throughout the genre. Placed and random encounters were both used in 1981s "Wizardry" and by the mid-1980s, random encounters made up the bulk of battles in genre-defining games such as "Dragon Warrior", "Final Fantasy", and "The Bard's Tale". Random encounters happen when the player is traversing the game world (often through the use of a "world map" or overworld). Most often, the player encounters enemies to battle, but occasionally friendly or neutral characters can appear, with whom the player might interact differently than with enemies. Random encounters are random in the respect that players cannot anticipate the exact moment of encounter or what will be encountered, as the occurrence of the event is based on factors such as programmed probabilities; Pseudo-random number generators create the sequence of numbers used to determine if an encounter will happen. The form and frequency can vary depending on a number of factors, such as where the player is located in the game world and the statistics of the player character. In some games, items can be found to increase or decrease the frequency of random encounters, even to eliminate them outright, or increase the odds of having a particular encounter.