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From Simple English Wiktionary[change source]
==Vertical in maths and science== === Mathematic use of the term "vertical" === :In Mathematics, the terms '''vertical''' and '''horizontal''' are always defined by referring to a particular point. * Although '''horizontal''' and '''vertical''' are [[direction]]s and really exist only as ideas or [[concept]]s, they can be shown by drawing lines on paper. The '''vertical''' line is drawn at right angles or [[perpendicular]] to the '''horizontal''' line * The terms '''horizontal''' and '''vertical''' can also be used for "planes" (flat surfaces), so the idea can be shown by putting two pieces of flat card at right angles (or perpendicular) to each other. ===Scientific use of the term "vertical'=== * The main scientific idea is that '''vertical''' and '''horizontal''' must always go together in science. So, if a scientist names one direction (or line) as '''horizontal''', then a direction (or line) that crosses it, on the perpendicular, must be named '''vertical'''. * This means that the ''real'' directions of '''vertical''' and '''horizontal''' are different in every different place. * On the surface of a planet, eg planet Earth, the idea of '''vertical''' and '''horizontal''' depends on gravity. For example: people in Australia, China and France all think of vertical as being a different direction, because the horizontal ground is a different direction. * This same idea of '''vertical''' and '''horizontal''' being related to gravity means that people floating weightlessly have no idea which way is "up".