In PowerPoint you can set various attributes for shapes — adjusting Transparency for Shape Fills is one of such tasks. Similar to the shape fills, shape outlines such as solid lines and gradient lines also have transparency attribute. This attribute lets you reduce the opacity of the outline (also known as the border or line), so that the slide object or background behind shows through partially. Transparency is calculated in percentages and you can change its value all the way from 0 to 100%.
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