Before a call to glFrustum, the z axis ranges fom -1 to 1, and is easy to handle.

However after a glFrustum call( glFrustum(-1,1,-1.2,1.2, 0.5,5) ) all of this changes. Suddenly values like 1.4 and above become valid. Quads begin intersecting when they are not supposed to. Can anyone please explain to me what is happening here?

How does znear and zfar of glFrustum map onto the z axis? Thanks for your help.