In strategy computer games, a technology, tech, or research tree is a hierarchical visual representation of the possible sequences of upgrades a player can take (most often through the act of research). 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.[1] Each level is called a tier and is often used to describe the technological strength of a player.