This corner elbow has a flat surface which allows PVC Pipe structures to sit flat on the ground.
Inspired by TonyYoungblood's 1/2" elbow, I created my version for larger diameter pipe and designed it to be tighter-fitting and use thinner glue.

The tighter allowances mean that supports will generally be necessary, however.

NOTE The corner is offset: The vertical member is not located at the axis defined by the other two.

I believe this produces a stable and pleasing form, but keep this in mind if you are creating PVC structure that has cross-braces in the same plane as the vertical members.

I may create an offset "T" in the future, but none of my structures have required this.