History: Why do we call the basic oscillators by "circle, triangle, square, rectangle, sawtooth"?

Why do we call the basic oscillators by "circle, triangle, square, rectangle, sawtooth"?

• The oscillators were named after their inventors.
  I've not heard of a ... Mr. Sawtooth ... before.
• The oscillators sound like these shapes.
  What does a shape sound like?
• There is no correlation, these are just names.
  There is a common reason for the naming.
• The oscillators visually look like these shapes.
  Good work.