The cost of a basic dc line is always less than that of an ac line. For submarine installations the difference is particularly pronounced; for underground cables, the difference is somewhat smaller. For overhead transmission on land, many studies show that a dc line generally costs about one third less than an ac line of the same capacity. It may be entirely justifiable, however, to compare a two-pole dc line with a double-circuit ac line that has two 3-phase circuits carried on the same towers. The dc line cost, on this basis, would be about 55 percent of the cost of the ac line.