"Convex" refers to the shape of the curve on top of the fence. If the the arc is higher in the center than at the posts it's convex. If the arc is lower in the center than at the posts it's concave.
The photo below shows a Billings in cedar with pyramid caps and rounded pickets, all straight with the grade.
|Wood Choices||Cedar or Pressure Treated|
|Bottom of Fence|
|Heights||24", 32", 48", or 72"|
|More||Other Wood Fence Styles|