Developed by Microsoft founders Paul Allen and Bill Gates, Microsoft BASIC, the first product ever licensed by the company, jumpstarted the personal computing revolution. This programming language interpreter allowed users of the Altair 8800 microcomputer to program in BASIC, an early programming language that was well known for its ease-of-use. The pairing of this software with the world’s first widely successful personal computer was a potent combination that cemented Microsoft’s relevance in the computer industry right from the company’s inception.

During the early days of computing, learning to program was extraordinarily difficult and inaccessible. Using a mainframe computer in the 1960s meant that you had to program your own software from scratch or commission others with the expertise to program custom software for your specific purposes. Often you were plugging in data one bit at a time by flipping switches on the front panel of your computer, sending ones and zeros—high and low electrical voltages—into your machine to trigger instructions. The inaccessibility of computer programming was a problem that two professors at Dartmouth College in the 1960s, John G. Kemeny and Thomas E. Kurtz, sought to grapple with. In 1964 they introduced a programming language called BASIC to students at Dartmouth as one of the earliest efforts to formalize computer education.

BASIC was written with an emphasis on ease-of-use. Its syntax closely resembles English, making it a handy entry point for beginner coders. It spread quickly throughout college campuses, its accessibility spurring a strong enthusiasm among young programming students. By the 1970s BASIC was firmly established as the standard mode of computer programming education. Well before their Microsoft days, Bill Gates and Paul Allen attended a small private high school in north Seattle called Lakeside High School where they began learning how to program in BASIC.

Enter: the MITS Altair 8800. Introduced in 1975, it became the first widely successful personal computer thanks largely to its unprecedented price-point: a modest $439. At the time of its release, Bill and Paul were living in the Boston area, Paul working at Honeywell and Bill starting his undergrad in pre-law at Harvard. They caught wind of the Altair one fateful day in December, 1974 when an issue of Popular Electronics with the system on the cover caught Paul’s eye. After he excitedly shared his discovery with Bill, the two of them hastily contacted Ed Roberts, the founder of MITS. They offered a white lie, claiming that they had written a version of BASIC for the 8080 microprocessor, ready to ship out in conjunction with the Altair. Ed Roberts, excited by this prospect, invited them out to Albuquerque to demo their software.

Now with a deadline, Bill and Paul scrambled to write the promised software, a programming language interpreter for the BASIC programming language, while facing a huge challenge: they didn’t have access to an Altair 8800, which at that point had been sold out for some time due to unforeseen demand. The two young programmers chose to emulate the Altair environment on a PDP-10 that they had access to, hoping that it would reliably run on the real thing. What’s more was that the typical Altair owner was running only 4K bytes of memory and fitting a BASIC interpreter into 4K bytes while leaving enough space to run a user’s programs was no easy feat. It took some very clever programming to write the most efficient software as possible.

Paul Allen was chosen to fly out to New Mexico to demo the finished interpreter for Ed Roberts. The pair were so strapped for time that it wasn’t until Paul got on the plane to Albuquerque that he realized that he neglected to include the bootstrap loader: a short, manually keyed program that is an essential part of the booting process. Doing calculations by hand on the plane, he managed to write one that would hopefully function properly once he had landed. Despite the lack of proper testing and the rushed development process, the demo with Ed Roberts went smoothly, leading to the founding of Paul and Bill’s software company for microprocessor computers which they dubbed Microsoft.

MITS and Microsoft struck up a deal, selling Altair 8800s in conjunction with the BASIC programming language interpreter which became known as Altair BASIC or alternatively, Microsoft BASIC. This pairing became the first instance of the modern software licensing model that we are familiar with today and ushered in a new era in the history of the computer industry. From this point forward it became standard that a certain piece of hardware was to be released paired with the software required to use it. This shift was made possible by the standardization of computer hardware in the age of the microprocessor where inter-compatibility between machines was a given.

With earlier minicomputers there were so many differences between systems that the newest iteration of your favorite computer would often render your current software obsolete. But the microprocessor made it so that Microsoft BASIC could run reliably on any computer based on the Intel 8080. And of course, after the Altair’s success, several clones of the system running on the same Intel 8080 microprocessor followed, making Microsoft BASIC a very valuable commodity. In the early days of Microsoft, the company was making most of its money from licensing software for 8080 machines. And as Altair BASIC proliferated, so did its piracy.

In 1976, Bill Gates penned the now-notorious “Open Letter to Hobbyists”, a disparaging response to the widespread unauthorized use of Microsoft’s Altair BASIC software among early computer hobbyists. The letter emphasized the importance of intellectual property, claiming that it was unfair for users to reap the benefits of software developers’ time and effort for free. Gates’ letter was published in several hobbyist magazines of the day and read aloud at meetings of the Homebrew Computer Club. The club was an early and highly influential computer hobbyist group that was one of the main targets of the letter. The letter produced tons of backlash from the Tiny BASIC project, an alternative to Microsoft BASIC that was collaboratively developed by hobbyists, and the inclusion of some pointed shade in a 1976 ad for the Apple I computer that stated “yes folks, Apple BASIC is free.”

Interest in BASIC persisted through the early 1980s as versions of Microsoft’s BASIC were ported to nearly every computer released during this time. However, the following generation of home computers that included machines like the IBM PC and the Apple Macintosh brought a sharp decline in the prominence of BASIC. Though most moved on to newer languages and operating systems emerging during this era, BASIC retained a following among dedicated hobbyists. Microsoft debuted Visual Basic in 1991, ushering in a resurgence in its use up until today. BASIC remains much less prominent than it was in its heyday, but it only takes a short walk through Living Computers’ vintage collection, noting iterations of BASIC on so many of our systems, to understand its unique place in the history of the computer industry.