In the statusbar,

* The Stroke indicator also displays the stroke width of selection (averaged if there are multiple objects selected with different stroke widths), located to the right of the stroke color/transparency swatch. Left-clicking on it opens the Fill&Stroke dialog with the Stroke Style tab selected. Right-clicking on it opens a popup menu which allows you to choose the units for displaying the stroke width, as well as choose one of the presets to assign to selection.

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