Since every garden is unique, and the best shape for your trellis is the one that works best in your garden and fits your gardening style. There are 2 basic trellis shapes: the A-Frame and the Box Frame, and there are many different versions of each of those.
A-Frame Trellis:
Stronger than Box Frame Trellis
More stable
Costs less
Easier to set up / take down
Allows for easier access to lower parts of plants
Box Frame Trellis:
Provides more climbing space than A-Frame Trellis
May be needed to avoid obstacles like vertical garden bed walls
