Technology tree

[[File:Freeciv-2.1.8 technology tree (cropped).png|thumb|upright=1.5|Part of Freecivs technology tree, showing complex dependencies between technologies. For example, astronomy can be reached by first researching ceremonial burial and then mysticism, or by researching masonry followed by mathematics]] In strategy games, a technology, tech, or research tree is a hierarchical visual representation of the possible sequences of upgrades a player can unlock (most typically representing the research progress of a given faction). Because these trees are technically directed and acyclic, they can more accurately be described as a technology directed acyclic graph'''. The diagram is tree-shaped in the sense that it branches between each 'level', allowing the player to choose one sequence or another. Each level is called a tier and is often used to describe the technological strength of a player. Typically, at the beginning of a session of a strategy game, a player will start at tier 1, and will only have a few options for technologies to research. Each technology that a player researches will open up one or more new options, but may or may not, depending on the computer game, close off the paths to other options. The tech tree is the representation of all possible paths of research a player can take, up to the culmination of said sequence. A player who is engaged in research activities is said to be "teching up", "going up the tech tree", or "moving up the tech tree". Analysis of a tech tree can lead players to memorize and use specific build orders. The Classic tech tree is the one where extensive research into new technologies must be conducted parallel to the progression of a game. Some real-time strategy (RTS) games as well as most turn-based strategy (TBS) games employ this type of tech tree. In these games, players typically have a 'command center', 'unit-training-facility' and 'research facility' at their disposal from the start, allowing them to both start research into more advanced technologies, or engaging in combat with enemies using the basic units (Warzone 2100, Mega Lo Mania, Master of Orion series, Civilization series, Space Empires series).