You can use triangle strips and triangle fans to specify an entire surface without having to provide all three vertices for each of the triangles. For example, only seven vertices are required to define the following triangle strip.
The system uses vertices v1, v2, and v3 to draw the first triangle, v2, v4, and v3 to draw the second triangle, v3, v4, and v5 to draw the third, v4, v6, and v5 to draw the fourth, and so on. Notice that the vertices of the second and fourth triangles are out of order; this is required to make sure that all of the triangles are drawn in a clockwise orientation.
A triangle fan is similar to a triangle strip, except that all of the triangles share one vertex.
The system uses vertices v1, v2, and v3 to draw the first triangle, v3, v4, and v1 to draw the second triangle, v1, v4, and v5 to draw the third triangle, and so on.
You can use the wFlags member of the D3DTRIANGLE structure to specify the flags that build triangle strips and fans.