In the 3D View header, to the right of where you choose whether you are in the Global or the Local mode, there are three boxes, as seen in the following screenshot. The left one has an orange dot next to a cube. It is the Vertex Select Mode button. The center one has an orange vertical line next to a cube. It is the Edge Select Mode button. The right one has an orange parallelogram on a cube. It is the Face Select Mode button. They control whether you are selecting vertices, edges, or faces.
Use the Tab key to get into Edit Mode in the 3D View, if you are not already.
In the 3D View window, click on one of the vertices of the cube with theRMB. Press the G key, and move the vertex. Press theRMB to release the vertex where it began.
Press the G key, and move the vertex. Press theLMB to release the vertex where you have moved it.
In the 3D View header, click theLMB over the Edge Select Mode button—the box with the orange vertical line.
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