In this era, Internet-mediated communication has become widespread. Increasingly, more and more people communicate through the Internet. The purpose of this research is to investigate the language used in written chat from multiple dimensions. The research was designed and conducted as a case study. A private high school in Kayseri / Turkey was selected as the case. Data were collected through various data collection instruments including surveys, interviews, and document review. According to the findings, students spend less than one hour for online chatting during schooldays. The time allocated for online chatting increases during holidays. A sign language specific to Internet-based communication with its own symbols and writing style is emerging. The language used by students during written chat has common features with daily spoken language. This language has various deviations and contains many examples of unique usage. Students follow almost none of the rules of written language.