| Zalman Usiskin and Jennifer Griffin, "The Classification of Quadrilaterals | Owen Byer, Felix Lazebnik and Deirdre Smeltzer, Methods for Euclidean Geometry, Mathematical Association of America, 2010, p |
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| Owen Byer, Felix Lazebnik and Deirdre Smeltzer, Methods for Euclidean Geometry, Mathematical Association of America, 2010, pp | A Study of Definition", Information Age Publishing, 2008, p |
Note: 's original definition and some English dictionaries' definition of rhombus excludes squares, but modern mathematicians prefer the inclusive definition.