Three-Centered Arch I: Drawing with a String and Three Nails

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The arch is made of three arcs of circles with centers at the solid green point and the two red points. The green string is attached at the lower green point (the first "nail") and the other end can be moved with the slider "green angle" around the two other fixed "nails" at the reddish points.
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Contributed by: Ralf Schaper (March 2011)
Open content licensed under CC BY-NC-SA
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Three-centered arches are often used as door or window arches. There are similar constructions for arches with four or five centers. The four-centered arch has a vertex at the highest point.
The mathematical background can be seen in the Demonstration "Three-Centered Arch II: Geometric Construction".
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