In the two-dimensional culture, "K1" usually refers to the servant. The term comes from sadomasochistic (S&M) culture, where the shape of the "1" is metaphorically represented by a person standing in service. Therefore, K1 represents the party who plays the role of a slave in an S&M relationship. The letter circle is the abbreviation of the S&M group and represents the English word sadomasochism. In the letter circle, K1 refers to the servant slave, while other related terms such as K2 stands for kneeling slave, K3 stands for torture slave, etc. These are named according to the roles played in the S&M relationship. The two-dimensional dimension originally refers to the "two-dimensional world", a two-dimensional space including length and width. Later, it became a specialized term in the ACGN subculture circle, and it was specifically referred to as "two-dimensional".