Say I have the top mesh in the image. If I subdivide it, I'll have the second mesh in the image. But what I want is the third. It has vertical subdivisions, but not horizontal. Is there a way to do this?
Tab into Edit mode, if you're not already there.
1. Select only the horizontal edges, then subdivide.
2. For each of the three sections, press CtrlR and hover the mouse near a horizontal edge, then LMB or Enter followed by RMB or Esc.