What are the extended homogeneous coordinates in the EdDSA specification? According to the EdDSA specification from the IETF:

For point addition, the following method is recommended.  A point (x,y) is represented in extended homogeneous coordinates (X, Y, Z, T), with x = X/Z, y = Y/Z, x * y = T/Z

I'm unfamilar with extended homogeneous coordinates, and I'm used to seeing points as simply (x, y).   Can anyone explain where the T and Z variables come from in the specification document?
UPDATE:
I found this answer on Stack Overflow that has some great detail:
Extended homogeneous coordinates are a different form of coordinates that provide particularly fast methods of point addition and point doubling formulas for twisted Edwards curves, introduced in Huseyin Hisil, Kenneth Koon-Ho Wong, Gary Carter, Ed Dawson. Twisted Edwards Curves Revisited, 2008.

$X$ and $Y$ roughly correspond to affine $x$ and $y$, $Z$ comes from regular extended (also called projective) coordinates as a scaling and $T$ is the actually “new” element that contains $X\cdot Y$. In particular, section&nbsp;3 of the paper by Hisil et al. introduces the extended homogeneous coordinates and how to convert to and from extended homogeneous coordinates.