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Topic: Two-Sided Regular Polygons
Replies: 9   Last Post: Jan 25, 2005 9:47 PM

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John Conway

Posts: 2,238
Registered: 12/3/04
Re: Two-Sided Regular Polygons
Posted: Feb 29, 1996 3:01 PM
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On 29 Feb 1996, Dan Bennett wrote:

> I'm asking this question for William Blackwell, who has yet to get
> connected but hopes to do so soon:
> A finite straight line has two sides and can be considered a regular
> n-gon where n = 2. (This line is also a rectangle with length, but 0
> width, an ellipse with one axis = 0, an isosceles triangle with base
> = 0, and so on. In every case it has two sides.) Is there a correct
> "gon"
> term (besides" line segment") for this shape? A bigon? duogon? digon?
> Thanks in advance for your help. William has lots of reasons to be
> interested
> in this shape which he looks forward to sharing when he gets online.

The only term of these three that is properly formed is "digon",
and in fact this term has often been used with this meaning.

"di" is Greek, and so is the natural prefix to mate with the
Greek-derived "gon", meaning "angle".

"bi" (abbreviating "bis") is the Latin equivalent - it mates for
example with Latin-derived "lateral" to give "bilateral",
meaning "two-sided", which you might also use in this
sort of context.
[I have sometimes seen the form "bigon", but regard it as incorrect.]

"duo" is a Latin combining form, used in making words like
"duodecimal" (= "12-ary").

The word "dihedron" is fairly well-accepted for the similar
degenerate polyhedron with just two faces, especially in its
adjectival form "dihedral".

John Conway

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