If you intend to do a lot of travel by subway, you should buy a value ticket. For 10,000 won, you get 11,000 won worth of travel. At the right, you see the front and back of a subway value ticket.
There are vending machines that sell value tickets, but the easiest way to get one is to go to the ticket counter at the station and say, "Value Ticket" and give them a 10,000 won bill.
It is possible to buy tickets for a single trip. Seoul is divided into "zones", and depending upon how many zones you are traveling through, the cost will be either 500 or 600 won (as of January 1999). Below are two types of ticket vending machines which you may use to purchase single tickets.
Subway Index