This unit deals with the most important geometric shapes on the plane: the triangle, the square, the rectangle, the parallelogram, the rhombus and the trapezium, and focuses on their area. As they are all two-dimensional we can say that they "live on the plane", unlike a matchbox (rectangular parallelepiped) which is three-dimensional and therefore "lives in three-dimensional space". Polygons are other kinds of plane shapes which you can find out about in the unit on Measuring Polygons.