This post is a feature request of something that I would like Adobe to improve upon. When creating borders around paths and shapes, wouldn't it be nice and convenient for us to be able to select or deselect the sides that we choose for the border?
Let's say you have a rectangle but only want a border on one side. We should be able to choose one side only by deselecting the other three sides. The work around for this is to draw another rectangle as the border for the one side. It works, but isn't as convenient.
To make this work the border option would have to work by creating a path around the object. Then we should be able to break the path by removing any sides where we don't want the border.
Maybe it's just me. I'm always looking for ways to improve a program. Food for thought.
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In point of fact, I'm a bit unclear as to exactly what you are looking for: adding a stroke to just one side of a polygon? Something else? Something visual might be helpful.