Drawing a diagram for each sum in this chapter is not always necessary, but it is often recommended although different students and teachers suggest that drawing a diagram greatly aids in problem-solving. Diagrams are used to envision the shape clearly and Mensuration is all about using your imagination. Drawing diagrams for every sum has also been noted to be one of the major causes that reduce avoidable mistakes in this chapter. What can be observed in the ICSE Class 9 Mathematics Revision Notes Chapter 21 is how much weightage this chapter holds and therefore everything that has been discussed can be great advice.