William Henry "Bill" Gates III was born in Seattle, Washington, on October 28, 1955 to William Henry Gates, Sir and Mary Maxwell Gates. His ancestry includes English, German, and Scots-Irish. His father was a prominent lawyer, and his mother served on the board of directors for First Interstate BancSystem and the United Way. Bill Gates's maternal grandfather was JW Maxwell, a national bank president. Bill Gates has one elder sister, Kristianne, and one younger sister, Elizabeth. He was the fourth of his name in his family, but was known as William Gates III or "Trey" because his father had the "II" suffix. Early on in his life, Bill Gates's parents had a law career in mind for him. When Bill Gates was young, his family regularly attended a Congregational church. The family encouraged competition; one visitor reported that "it didn't matter whether it was hearts or pickleball or swimming to the dock ... there was always a reward for winning and there was always a penalty for losing".
At 13 he enrolled in the Lakeside School, an exclusive preparatory school. When he was in the eighth grade, the Mothers Club at the school used proceeds from Lakeside School's rummage sale to buy a Teletype Model 33 ASR terminal and a block of computer time on a General Electric (GE) computer for the school's students. Bill Gates took an interest in programming the GE system in BASIC, and was excused from math classes to pursue his interest. He wrote his first computer program on this machine: an implementation of tic-tac-toe that allowed users to play games against the computer. Bill Gates was fascinated by the machine and how it would always execute software code perfectly. When he reflected back on that moment, he said, "There was just something neat about the machine." After the Mothers Club donation was exhausted, he and other students sought time on systems including DEC PDP minicomputers. One of these systems was a PDP-10 belonging to Computer Center Corporation (CCC), which banned four Lakeside students - Bill Gates, Paul Allen, Ric Weiland, and Kent Evans - for the summer after it caught them exploiting bugs in the operating system to obtain free computer time.
At the end of the ban, the four students offered to find bugs in CCC's software in exchange for computer time. Rather than use the system via Teletype, Bill Gates went to CCC's offices and studied source code for various programs that ran on the system, including programs in Fortran, Lisp, and machine language. The arrangement with CCC continued until 1970, when the company went out of business. The following year, Information Sciences, Inc. hired the four Lakeside students to write a payroll program in Cobol, providing them computer time and royalties. After his administrators became aware of his programming abilities, Bill Gates wrote the school's computer program to schedule students in classes. He modified the code so that he was placed in classes with "a disproportionate number of interesting girls." He later stated that "it was hard to tear myself away from a machine at which I could so unambiguously demonstrate success." At age 17, Bill Gates formed a venture with Allen, called Traf-O-Data, to make traffic counters based on the Intel 8008 processor. In early 1973, Bill Gates served as a congressional page in the US House of Representatives.