spend resources on upgrades or research. Some of these upgrades enhance the player's entire economy. Other upgrades apply to a unit or class of units, and unlock or enhance certain combat abilities. Sometimes enhancements are enabled by building a structure that enables more advanced structures. Games with a large number of upgrades often feature a technology tree, which is a series of advancements that players can research to unlock new units, buildings, and other capabilities. Technology trees are quite large in some games, and 4X strategy games are known for having the largest. A build order is a linear pattern of production, research, and resource management aimed at achieving a specific and specialized goal. They are analogous to chess openings, in that a player will have a specific order of play in mind, however, the amount of the build order, the strategy around which the build order is built or even which build order is then used varies on the skill, ability and other factors such as how aggressive or defensive each player is. Map and exploration Early strategy games featured a top-down perspective, similar in nature to a board game or paper map. Many later games adopted an isometric perspective. Even with the rise of 3D graphics and the potential to manipulate the camera, games usually feature some kind of aerial view. Very rarely do strategy games show the world from the perspective from an avatar on the ground. This is to provide the player with a big-picture view of the game world, and form more effective strategies. Exploration is a key element in most strategy games. The landscape is often shrouded in darkness, and this darkness is lifted as a player's units enters the area. The ability to explore may be inhibited by different kinds of terrain, such as hills, water, or other obstructions. Even after an area is explored, that area may become dim if the player does not patrol it. This design technique is called the fog of war, where the player can see the terrain but not the units within the explored area. This makes it possible for enemies to attack unexpectedly from otherwise explored areas. Real-time versus turn-based Strategy video games are categorized based on whether they offer the continuous gameplay of real-time strategy, or the discrete phases of turn-based strategy. These differences in time-keeping lead to several other differences. Typically, turn-based strategy games