K-1 visa if for a fiance of a U.S. citizen.A fiance is not allowed to come to the U.S. with a visitor's visa if their intent is to marry a U.S. citizen and apply for a green card in the USA. The correct procedure is to apply for K-1 which is a fiance Visa. Then the person may come to the U.S. with the K-1, get married and apply for a green card.

If you follow the K-1 procedure it minimizes the chances of having a problem at the green card interview

First a petition must be approved by the Immigration Service in America.

Second the fiance must be interviewed and receive the K-1 visa.

Third, the fiance comes to the U.S. and gets married within 90 days.

Fourth, apply for permanent residency. In most cases, the green card will be good for two years.

Fifth within 90 days of the expiration of the first green card, apply for a permanent card.

The K-1 visa may be used for only one entry. If the fiance leaves the U.S. without getting married you have to reapply from the beginning.