The following mental exercise may illustrate the meaning of topographic prominence. Imagine a peak and imagine that an imaginary sea level (based on an elevation above the geoid) rises to the peak. Now slowly lower the imaginary sea level and an imaginary island appears beneath your feet. The island will grow and will merge with other islands that emerge. Eventually, the island will touch an island with a higher peak than the initial island (i.e., an imaginary island that existed before lowering the imaginary sea level.) The summit of that island is the parent peak of the summit, the point at which the two islands touch is the key col of the summit, and the elevation rise from the key col to the summit is the topographic prominence of the summit.