In the late 1970s and early 1980s, computer vendors and software-only companies began routinely charging for software licenses, marketing software as "Program Products" and imposing legal restrictions on new software developments, now seen as assets, through copyrights, trademarks, and leasing contracts. In 1976 Bill Gates wrote an essay entitled "Open Letter to Hobbyists", in which he expressed dismay at the widespread sharing of Microsoft's product Altair BASIC by hobbyists without paying its licensing fee. In 1979, AT&T began to enforce its licenses when the company decided it might profit by selling the Unix system. In an announcement letter dated 8 February 1983 IBM inaugurated a policy of no longer distributing sources with purchased software.