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What's that embossed or raised part of an arc or arch called? I am looking for the upper part of the shape, which is kind of raised and forms a mini circle.

enter image description here

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Do you mean an arc like the geometric shape? Or an arch, like the Arc de Triomphe? –  simchona Aug 5 '12 at 23:55
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IF you mean an arch, you may be referring to the keystone. However, any embossment would be purely decorative and not an integral part of the keystone. –  Jim Aug 6 '12 at 0:36
    
You mean the arched part of the arch? –  The Photon Aug 6 '12 at 5:56
    
@ThePhoton- Yea, but only the upper side of it. Not the whole curve. –  Noah Aug 6 '12 at 5:57
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I think this question can safely be labeled Noah's Arc :)) –  Luka Ramishvili Aug 6 '12 at 7:14
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2 Answers 2

up vote 19 down vote accepted

I don't see a "circular" bit at the top of the arch in the OP's picture.

The parts of the arch are:

  • The absolute top of the arch is the apex.
  • The curve at the top of the arch is known as the crown.
  • The point at which the curve begins is the springing or spring-line.
  • The curve between the springing and the crown is known as the haunch.
  • The inner curve of the arch is the intrados.
  • The outer curve is the extrados.
  • The arch itself is made of sections called voussoirs. So the crown and the haunch are made of a number of voussoirs.
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Perhaps ogive, of which wikipedia says:

An ogive (/ˈoʊdʒaɪv/OH-jyv) is the roundly tapered end of a two-dimensional or three-dimensional object.

Later, the wikipedia article on ogives distinguishes between "spitzer ogives" and "elliptical ogives". A Gothic arch typically is a spitzer ogive, while the Gateway Arch illustrated in the question is a "weighted or flattened" catenary arch. A catenary, wikipedia says, is "superficially similar in appearance to a parabola (though mathematically quite different)". At certain magnifications, portions of parabolas, ellipses, and catenaries look much alike, and I think the top of the Gateway Arch looks a lot like an elliptical ogive.

Wiktionary lists several senses of ogive, including "A Gothic pointed arch, or a rib of a Gothic vault" and "(ballistics) The pointed, curved nose of a bullet, missile, or rocket".

You may also find terms like the following of interest: archivolt (or voussure), "an ornamental molding or band following the curve on the underside of an arch"; spandrel; intrados, "inner curve of an arch or vault"; voussoir, "a wedge-shaped element, typically a stone, used in building an arch or vault". See nomenclature in first diagram in wikipedia arch article.

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