From BlenderWiki

Creating Faces and Edges

This is a very commonly used tool, as it creates edges from vertices, and faces from vertices and edges.

To create an edge, you must have exactly two vertices selected. If they already form an edge, nothing will happen, else a new edge will be created between the two vertices.

Face creation is a bit more complex. First, let’s define what is a “convex perimeter”: for four or more vertices, they define a convex perimeter if you can find a way to link them by edges drawing a loop line where all internal angles are inferior to 180°. This is an important concept, as convex perimeters will always give “good” faces, i.e. faces that do not “overflow”. Note that in Blender, this mainly concerns quads (i.e. groups of four vertices), as triangles are always convex, and FGons are always built from tris and quads.

Face creation is enabled when you call it with three or more selected vertices, and behaves differently for different cases:

• If only three vertices are selected – this is the most simple case –, a triangle face will be created if none already existed. Edges will be also implicitly created if needed.
• If four vertices are selected, there are several sub-cases:
  • If less than four edges link the vertices, if the four vertices define a convex perimeter, edges and quad are directly created; else, nothing occurs.
  • If the vertices are already linked by exactly four edges, a quadrangle is always created. Note however that if the edges do not define a convex perimeter (or if they cross themselves), you will likely get a face overflowing its borders.
  • If the vertices are already linked by exactly five edges, two triangles will be created. This allows you to avoid overflowing faces with non-convex perimeters.
  • If the vertices are already linked by exactly six edges, four triangles will be created. This in fact creates a volume (a tetrahedron or “triangular-based pyramid”), with four faces. Beware that if your four vertices are in the same plane, you will get a flat tetrahedron!
• If five or more vertices are selected, the tool will pop-up a small Make Faces menu, with following choices:

Auto

This will create all faces (triangles and quadrangles) that are completely “drawn” by selected edges (i.e. where edges form a complete perimeter).

Make FGon

This will create FGons from all groups of selected linked faces. If no face is selected, you will get the error message “No faces selected to make FGon”.

Clear FGon

This will clear all selected FGons (i.e. turn them back into original triangles and quads).

Skin Faces/Edge-Loops

This is an advanced option, that “skins” (i.e. creates faces between) edge loops. It will prompt various error messages when the selected elements do not fulfill its conditions…

About face creation, see also the Fill tools description.

Once you have created all faces you need, you will likely get wrong normals in your mesh (they show up as black lines in shaded views/rendering), so you’ll have to recalculate them.