In the Chamfer command, if you select an edge of a solid at the Select first line prompt, the AutoCAD software immediately prompts you to select a base surface and highlights one for you to accept or reject. The base surface is a plane that touches the edge you have selected. Every 3D edge has two possible base surfaces. The AutoCAD software suggests one by highlighting it. You can toggle between the two surfaces by using the Next option at the Enter surface selection option prompt.