Slang is language that takes off its coat,
spits on its hands, and goes to work.

Concept 4 : Viewport Transformation 
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Idea
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void glViewport(x, y, width, height); defines in pixel
units the window in which the image is drawn.  This drawn
window is part (maybe the whole) of the window opened
by the windowing system on the terminal screen.  'x' 'y' 
define the lower left corner of the viewport, and width
and height describe the size.

Experiment:
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Move your mouse over the window showing the solitary 
cube, and click anywhere. Now press the number '2' on the 
keyboard.  We see two cubes.  

Depress the key 3 on the keyboard 3.  Now you should see 
three cubes rendered in three different viewports.

Question:
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With multiple viewports note the change in the aspect
ratio though.  Why do you think this happens?

Observations
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The aspect ratio of a viewport should generally be equal 
to the aspect ratio of the viewing volume.  If the two 
ratios are different, the projected image will be distorted 
when mapped to the viewport as seen in our examples.  
Adjust the aspect ratio of the projection transformation to 
match the viewport to overcome this problem.