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A few lines added to the square converts it into a cube. The extra lines suggest the third dimension to show depth of the subject. In these sketches of cubes, you can visualize three sides. Keep in mind that this is only an illusion.
Likewise with a few additional lines, the rectangle can become a cylinder. The circle can also be used to develop a cylinder. In the case of the circle, the circle is changed into an oval. The oval may be also called an ellipse.
The cone may be developed from the ellipse or triangle.
The circle becomes a sphere with a few additional lines added to suggest depth and width. With these basic forms you can combine them in a number of ways to create other forms the represent things that you are familiar with. Practice these basic forms and try to change them into other objects.
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