In Vietnamese mathematics curricula, primary school students explicitly learn the concept of quadrilaterals such as parallelogram, rhombus, rectangle, square and trapezoid in the Grades 3, 4 and 5.  They are presented individually, and there is no comparison between their characteristics. Therefore, the students will be difficult to recognize the relationships among kinds of quadrilaterals. The results of an investigation of 186 primary school students revealed that most of them found it easy to identify squares and rectangles but many of them asserted that “a square is not a rectangle”.