Plan your pipe fittings
I found it helpful to draw (and redraw) my proposed greenhouse. Only through this kind of visualization was I able to build my inventory list.
For this project you will need many different kinds of PVC pipe fitting. For my project it looked like
24 - 45 degree elbos 3/4 inch
4 - cross pipe fitting 3/4 inch
2 - T fittinggs 3/4 inch
24 - T fittings for 3/4 inch -1 inch
24 - 90 degree elbow 1 inch
4 - T fitting 1 inch
Remember that the T-pipe fittind along the base ahve to be "3/4 inch -1 inch T pipe fittings". This indicates that a 3/4 inch pvc pipe (the rib) will be joined to 1 inch PVC pipes (the base)
Buy a few more than you need of all sizes
They are cheap and it will allow for mistakes and adjustments along the way. Keep them in good shape and return the extras. T's are especiall helpful because you add a cross-support anywhere in your square structure where you want a bit more firmness.
Don't worry about the above counts. I will post a complete project manifest once my Greenhouse is complete