About Joining Mesh Elements
You can join two or more mesh elements of a control mesh to define the geometry quickly. You must select the pair of mesh elements that you want to join. The first selection stays static whereas the second selection moves to join the first selection. Thus unlike the operation of connecting mesh elements, new geometry is not created when you join mesh elements. You can join the following pairs of mesh elements of shapes:
Faces or edges one or more shapes
Same number of contiguous faces or edges
Vertices of an edge
A single edge
A single face
You can join only one-sided edges.
You cannot join mesh elements on which multilevel subdivision has been performed.
When you select a single edge, a single face, or vertices of an edge to join, they are converged into a single vertex.
When joining a closed loop of edges or faces, select a pair on each side in the order in which you want to join them.