If you have ever tried to make big things that require they be made in parts, you have probably run into this problem: the join points show. Look at the example in the lower right of the image (2 arrows). The desired result is to the left (single arrow).

Image by by Drongle McMahon – Arrows by me.

This is not a matter of getting the vertices to match. Even if they do match perfectly, you get shading errors in Second Life. This is a problem with vertex normals.

There is an ongoing thread about this problem in the SL Forum: Visible seam when connecting two smooth shaded pieces. A good explanation of how to achieve a smooth surface at the connection point with correct shading is given by Drongle McMahon here.