Seen on Seoul Subway Line 8.

There have been several ways of Romanizing the Korean language, though every system has its weaknesses. Among them, the McCune-Reischauer System, named for the two American linguists who created it in 1937, is the one used by the international community as well as the United States government and military; South Korea itself used it between 1984 and 2000, primarily to accommodate the needs of foreign visitors to the 1988 Seoul Summer Olympic Games. It replaced the Ministry of Education System which had used a one-to-one correspondence of each Korean vowel/consonant with a Roman letter, which often made for unpronounceable names (i.e. Mt. Seorak on the east coast would be Mt. Seolag under Ministry of Education).

One key feature of McCune-Reischauer is the usage of aspirated consonants for initial unaspirated consonants. In other words, a consonant that would normally be written as a J would be written as CH if initial; similarly, a G would become a K when initial, and a D turned to a T. The reason for this is that unaspirated consonants, when initial, do get a slight bit of aspiration; this is the same reasoning used by the Wade-Giles Romanization of Chinese.

Under that system, Jamsil, the neighborhood that hosted the Olympiad in 1988, is written as Chamshil, and that is the Romanization seen on this sign. The name Chamshil is indeed familiar to the foreign visitors who came for the Olympics. Every name was re-Romanized at that point (Busan became Pusan, for example), except for Seoul, which should have been revised to Sŏul but kept its historical Romanization. As McCune-Reischauer concentrates on actual pronunciation with no regard for how the word is spelled in Korean, it is hated by the Koreans themselves as much as it is favored by foreigners.

As a result, since 2000, South Korea has used Revised Romanization, which is mostly a revival of the pre-1984 Ministry of Education System, but with some special rules to assist with correct pronunciation taken from McCune-Reischauer. The initial consonant rule is no longer used, so Chamshil, which used to be Jamsil pre-1984, is again Jamsil. The Revised Romanization is the official system used by South Korea today, including all signs (except for this one which somehow had escaped conversion), and it is the system I use for my post-2000 South Korean photos.