While smaller groups within the Alps may be easily defined by the passes on either side, defining larger units can be problematic. A traditional divide exists between the Western Alps and the Eastern Alps, which uses the Splügen Pass () on the Swiss-Italian border, together with the Rhine to the north and Lake Como in the south as the defining features. While the Splügen Pass is neither the lowest nor the most important pass in the Alps, it is approximately halfway along the main chain, and makes a convenient boundary.