在几何学中，共面是指几何形状在三维空间中的一种关系。

共面，又称为共平面，是指几何形状在三维空间中落在同一平面上的关系。一般三个点必会共面，而四个点不一定会共面，两条平行直线必共面。

• 三个点必会共面
• 一条直线和这直线外一点必共面
• 两条直线相交，则它们必共面
• 两条平行直线必共面

In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. However, a set of four or more distinct points will, in general, not lie in a single plane.