SketchUp Select Tool: Expanding the selection set
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Expanding the selection set using the mouse
You can automatically add to a selection set by clicking the mouse button (while in the Select tool) multiple times in rapid succession. Click once on an entity to select that entity. Click rapidly twice (double-click) on an entity, namely an edge or face, to select corresponding faces or edges respectively. Click three times (triple-click) on an entity, namely an edge or face, to select the edge or face and all entities physically connected to that edge or face. The following image shows this click/selection sequence.