Just like a curve is the basic building block for figures in a plane, a surface is the basic building block for figures in space. A surface is essentially a curve with depth. Curves and surfaces are analogous in many ways. If you think of a curve as being the trace of the motion of a point in a plane, a surface is like the trace of the motion of a curve in space. Surfaces are continuous, meaning that given two points on a surface, you can start from one and reach the other without leaving that surface. Just like a curve is still one-dimensional, a surface, although it exists in three dimensions, is still two-dimensional. For example, when you build a curve by tracing the motion of a point, that curve, although it spans both length and width, has no width of its own. The curve doesn't have area, it only has length, one dimension. Similarly, a surface can span more than one plane, but it still does not have depth of its own. It only has two dimensions, length and width. We will work mostly with the simplest surface, a plane. Below various surfaces are pictured.

Figure %: Surfaces in space

Surfaces can be classified as being closed or simple closed surfaces. The surfaces that form the boundaries of geometric solids are simple closed surfaces, so we'll focus on them. A simple closed surface is one that divides space into three distinct regions:

- The set of all points inside the surface (the interior of the surface).
- The set of all points outside the surface (the exterior of the surface).
- The set of all points on the surface.

A simple closed surface can also be either convex or concave. The rules are very similar to those we saw in Polygons. A convex surface is one in which any two points on that surface can be joined by a segment that lies either on the surface or in the interior of the surface. A concave surface has a segment between points on the surface that lies in the exterior of the surface.

One more note on surfaces: a surface, even if it is a simple closed surface,
*does not* include the space in its interior. When a simple closed surface
is united with its interior points, it is no longer a surface, it is a geometric
solid.

So far we've only discussed parallelism and perpendicularity with respect to lines, but planes can be parallel and perpendicular, too. To understand relationships between planes, one must understand relationships between lines and planes.

A line and a plane are parallel if and only if they do not intersect. A line
*l* and a plane are perpendicular if and only if the line *l* is
perpendicular to every line in the plane that contains the intersection point of
line *l* and the plane. These cases are drawn below.