borderwidth

Borderwidth describes the thickness of the border surrounding an element. In web design this thickness is controlled mainly by the CSS property border-width, which can apply to all sides of an element or to individual sides (top, right, bottom, left).

Border width accepts length values such as 1px, 0.5em, or keyword values thin, medium, and thick. A border width value only takes effect if a border style is defined (solid, dashed, dotted, double, etc.). If the border-style is none or hidden, changing the width has no visible effect.

Shorthand syntax allows specifying widths for multiple sides at once, for example border-width: 2px or border-width: 2px 4px 2px 4px.

In the CSS box model, the border is drawn outside the element's content and padding, and its width contributes to the element's total size. If the box-sizing property is border-box, the border width is included in the element's declared width; otherwise it adds to the overall size.

Beyond CSS, the term borderwidth appears in GUI toolkits such as Tkinter, where a widget parameter named borderwidth or bd defines the thickness of the widget's border. The exact behavior varies by framework. In HTML, the deprecated border attribute on tables is superseded by CSS border-width. See also CSS border and the CSS box model for related concepts.