Math Forum http://mathforum.org/kb List of forum topics en Re: Classifying quads visually http://mathforum.org/kb/thread.jspa?messageID=6482383&tstart=0#6482383
Another interesting approach to classifying quadrilaterals appears in the paper by Branko Grünbaum:

The Angle-side]]> Nov 1, 2008 11:31:48 AM Nov 1, 2008 11:31:48 AM malkevitch@york.cuny.edu 0 Re: Classifying quads visually http://mathforum.org/kb/thread.jspa?messageID=6329231&tstart=0#6329231
I also have a visual classification of quadrilaterals. It is interactive, too. It can be found]]> Aug 9, 2008 12:24:06 AM Aug 9, 2008 12:24:06 AM alexb@cut-the-knot.com 0 Classifying quads visually http://mathforum.org/kb/thread.jspa?messageID=6321713&tstart=0#6321713 Aug 4, 2008 4:04:01 AM Aug 4, 2008 4:04:01 AM profmd@mweb.co.za 2