Editing Up to (section)

===Tetris===
[[File:Tetrominoes IJLO STZ Worlds.svg|thumb|Tetris pieces I, J, L, O, S, T, Z]]
Consider the seven [[Tetris]] pieces (I, J, L, O, S, T, Z), known mathematically as the [[tetromino]]es. If you consider all the possible rotations of these pieces — for example, if you consider the "I" oriented vertically to be distinct from the "I" oriented horizontally — then you find there are 19 distinct possible shapes to be displayed on the screen. (These 19 are the so-called "fixed" tetrominoes.<ref>{{MathWorld |id=Tetromino |title=Tetromino |access-date=2023-09-26}}</ref>) But if rotations are not considered distinct — so that we treat both "I vertically" and "I horizontally" indifferently as "I" — then there are only seven. We say that "there are seven [[tetromino]]es, up to rotation". One could also say that "there are five tetrominoes, up to rotation and reflection", which accounts for the fact that L reflected gives J, and S reflected gives Z.