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By default, polyline segments have the default line thickness (which can be set in the Pen Width box on the Property toolbar.)
For any segment, you can enter values for Start and End Width.
The next segment will have these properties, but the subsequent segments, by default, will revert to the default thickness.
To change the widths of adjacent segments you can open the Polyline Properties from the local menu or Inspector Bar.
In this window, you can force start and end widths to be equal, and to set the start width of each segment to be equal to the end width of the previous segment.
Activate both options if you want to apply a uniform thickness to the entire polyline, or to a contiguous group of segments.
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