Joined but not connected

I know there must be something simple here that I am missing but I am working on a project where I had separate objects for the purpose of UV mapping and then joined them together only to find the vertices were not connected at the area joined. This was one complete object, no ngons and all clean. I only separated the object for the purpose of UV mapping but when I joined them together again, I noticed the areas where the objects were joined did not merge the vertices. What am I missing here?

To get the overlapping verticies to merge, select all and then run the “remove doubles” command. The Join command does not automatically remove doubles, because sometimes the meshes should remain separated, and it’s a lot harder to get them apart than it is to merge them.

I didn’t know that. Thanks for the tip

You should not need to split the object up for UV mapping. UV mapping should be done marking edges as seems(ctrl+e -> a when edges are selected) and unwrapping (u -> u with faces to unwrap selected). You can also pin UV’s in the UV editor (p to pin selected UVs or alt+p to unpin) so they are not moved when unwrapping. That is useful if for example you wish the borders of a UV island to remain as you shaped them but the inside to be recalculated to match the new shape.