When we draw a solid on paper we are making a 2-D representation of a 3-D object.
Here are two pictures of the same cuboid, measuring 4x3x2 units.
|cuboid on ordinary squared paper and on isometric paper|
The dimensions of the cuboid cannot be taken from the first picture but they can be taken from the picture drawn on isometric paper. Instead of isometric paper you can also use 'triangular dotty' paper.
|triangular dotty paper|
Be careful to use it the right way round.