Template:Blender 3D: HotKeys/3D View/Edit Mode/FKEY

FKEY - creates segment/edge/face. If two vertices are selected, create an edge connecting the two vertices. If three or four vertices are selected, or two edges are selected, create a face connecting the vertices or edges. If two co-planar faces are selected, join the faces to create an FGon, or dismantle a previously created FGon.