# Triangle with a Hole **URL:** \ **Category:** Factual Questions\ **Created:** [September 1, 2003, 8:17pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011 "2003-09-01T20:17:43Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![SuperNova](https://avatars.discourse-cdn.com/v4/letter/s/ea666f/32.png) [@SuperNova](https://boards.straightdope.com/u/SuperNova)\ **Post date:** [September 1, 2003, 8:17pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/1 "2003-09-01T20:17:43Z")
Not sure exactly what to make of [this,](http://www.ebaumsworld.com/trigrid.html) Can anyone give me an explanation? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [September 1, 2003, 8:23pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/2 "2003-09-01T20:23:51Z")
The second one isn’t a triangle. [How does this triangle puzzle work?](http://boards.straightdope.com/sdmb/showthread.php?threadid=204404&highlight=triangle) [triangle puzzle](http://boards.straightdope.com/sdmb/showthread.php?threadid=114471&highlight=triangle) [Hey, what’s going on with this triangle (geometry question)?](http://boards.straightdope.com/sdmb/showthread.php?threadid=89065&highlight=triangle) (I can’t believe **ultrafilter** fell for it. Hehehehe.) ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [September 1, 2003, 8:25pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/3 "2003-09-01T20:25:51Z")
That’s an oldie, **SuperNova**. I just can’t remember how it works, though. Someboby will. Peace, mangeorge ---
**Author:** ![SuperNova](https://avatars.discourse-cdn.com/v4/letter/s/ea666f/32.png) [@SuperNova](https://boards.straightdope.com/u/SuperNova)\ **Post date:** [September 1, 2003, 8:30pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/4 "2003-09-01T20:30:18Z")
> [@](#): > > \*Originally posted by friedo \* > \*\*The second one isn’t a triangle. > > [How does this triangle puzzle work?](http://boards.straightdope.com/sdmb/showthread.php?threadid=204404&highlight=triangle) > > [triangle puzzle](http://boards.straightdope.com/sdmb/showthread.php?threadid=114471&highlight=triangle) > > [Hey, what’s going on with this triangle (geometry question)?](http://boards.straightdope.com/sdmb/showthread.php?threadid=89065&highlight=triangle) (I can’t believe **ultrafilter** fell for it. Hehehehe.) \*\* Thanks ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [September 1, 2003, 8:30pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/5 "2003-09-01T20:30:49Z")
> [@](#): > > \*Originally posted by friedo \* > The second one isn’t a triangle. You’re right, of course, but the first one isn’t a triangle either. ---
**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\ **Post date:** [September 1, 2003, 8:55pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/6 "2003-09-01T20:55:49Z")
As a side question, what _is_ it called? A polygon? (don’t those have to have straight sides?) ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [September 1, 2003, 9:05pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/7 "2003-09-01T21:05:31Z")
> [@](#): > > \*Originally posted by DarrenS \* > \*\*As a side question, what _is_ it called? A polygon? (don’t those have to have straight sides?) \*\* That quasi-triangle _does_ have straight sides. It has four sides, and the angle between the two that appear to form the hypotenuse is just under 180°. Technically, the figure in question is a quadrilateral. ---
**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\ **Post date:** [September 1, 2003, 9:15pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/8 "2003-09-01T21:15:58Z")
That hypotenuse looks like a curve to me, not a straight line or even two lines meeting at an obtuse angle. ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [September 1, 2003, 9:30pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/9 "2003-09-01T21:30:11Z")
I’m fairly certain that the apperance of being curved is an optical illusion caused by the shallow angle at which the two small triagles meet. Of course, I don’t have a flat monitor, so maybe I’ve been conditioned to view shallow curves as straight lines. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [September 1, 2003, 9:39pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/10 "2003-09-01T21:39:19Z")
Yeah, they got me. Happens every once in a while. Depending on the definition you use, that figure may be a polygon (although non-convex). A friend of mine who did some research with computer vision did use the more general definition. ---
**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\ **Post date:** [September 1, 2003, 9:40pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/11 "2003-09-01T21:40:58Z")
OK - so assuming it is a curve, what is the name for that triangle-like figure? It’s possible there’s no name for it (we don’t name every single possible figure) - perhaps mathematicians are content to lump it in with “figures that are topologically equivalent to a circle.” ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [September 1, 2003, 10:01pm UTC](https://boards.straightdope.com/t/triangle-with-a-hole/199011/12 "2003-09-01T22:01:02Z")
Semi-circle?