(default keyboard shortcut Shift Z)
Detail > Polygons > Merge Polygons joins two selected polygons that share at least one common edge. A common edge is one where two or more polygons share all of the points along that edge.
The Merge Polygons command also works on curves, if they share a common terminating point. It doesn’t matter if you use the start or end of the curve. They will be merged into one continuous curve.
Common problems are edges that don’t share all points. Try merging points first to eliminate overlapping points. After using this command, check for non-planar polygons, which often result. Another issue with both merging points or polygons is the presence of Morph maps. These will prevent merges.
Merge Polys can also be used for merging splines.