Isometric drawing is a pictorial representation of an object in which all three dimensions are drawn at full scale rather than foreshortening them to the true projection. An isometric drawing looks like an isometric projection but all its lines are parallel to the three major axes.

Oblique drawings show a three dimensional view of an object. The width of the object will still be drawn as a horizontal line, but the depth can be drawn back at any angle. There are three types of obliques, cavalier, normal and cabinet obliques. Cabinet obliques are when you cut the depth in half. Cavalier obliques are when you keep the depth the full measurement. Normal obliques are when you cut the depth into three quarters.

Orthographic drawings show a three-dimensional object in two dimensions. It is a form of parallel projection, where the view direction is orthogonal to the projection plane resulting in every plane of the scene appearing in affine transformation on the viewing surface.

So the difference between these drawings is that isometric drawing is when all three dimensions of an object are drawn at full scale rather than foreshortening them to the true projection while in oblique drawings, the width of the object will still be drawn as a horizontal line, but the depth can be drawn back at any angle and orthographic drawings show a three-dimensional object in two dimensions.

These drawings are isometric, oblique and orthogonal respectively.

Sources:

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