Manipulated Forms: Stretched, squeezed, twisted, wrapped, added, subtracted forms that can be used as base building blocks of more complex forms. For more on this, see this guide.
3D model of an opened chest, drawn with construction lines to show how to construct it. The idea for this model came from a video thumbnail on Proko introducing their perspective course.
This is a simplified 3D model for this chess pawn. You can practice drawing the simplified 3D form first before tackling the photo and start drawing the pawn rotated, as is demonstrated here. The model consists of cylinders and a sphere.