Isometric drawings are 3D drawings. They show three sides, all in dimensional proportion, but none are shown as a true shape with 90 degree corners. All the vertical lines are drawn vertically but all horizontal lines are drawn at 30 degrees to the base line. Isometric is an easy method of drawing 3D images.
Oblique drawings are also used in engineering. The object is drawn with the most distinguishing features facing directly towards the observer, showing the true shape of these features. Circular features like round holes can be drawn on this front face as true circles. In other 3D forms, circular features must be drawn as ellipses.
To show the 3D effect, parallel lines (called 'lines of sight') are drawn from the front face at an angle.
The cavalier method is the simplest form of oblique projection.
All features in the oblique projection view are drawn to the same scale eg 1:1, and the lines of sight are drawn at 45° to the horizontal as shown in the diagram. The lines of sight may appear to diverge excessively and so the shape may appear to be very much. Other methods of oblique proportion try to reduce these distorting effects, usually by reducing the scale used along the lines of sight to half that used for the features facing the observer to create the appearance of depth.