The most common case occurs in naive implementations of continuous level of detail, where a finer-level mesh is "sewn" together with a coarser-level mesh by simply aligning the finer vertices on the edges of the coarse polygons.
The most typical scenario arises in uncomplicated implementations of continuous level of detail, where a finer-level mesh is "stitched" together with a coarser-level mesh by aligning the finer vertices on the edges of the coarse polygons.