glOrtho vs. glFrustrum

Even though I own the "Red Book" and have read the description of the two thoroughly, I still have trouble deciding which to use for which circumstances.

Can anyone help me out?

[Pauly]

Ortho creates a parallel projection - IE one with NO perspective effects. Thats useful for CAD, undistorted views of elevations, that kinda thing.

Frustrum creates, err, a frustrum - like a pyramid on its side with the end chopped off. That's what you'd use when you wanted a perspective effect.

gluPerspective wraps up glFrustrum nicely...

Paul.