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Gratis versus libre
The English adjective free is commonly used in one of two meanings: “at no monetary cost” (gratis) and “with little or no restriction” (libre). This ambiguity of free can cause issues where the distinction is important, as it often is in dealing with laws concerning the use of information, such as copyright and patents.
The terms gratis and libre may be used to categorize intellectual property, particularly computer programs, according to the licenses and legal restrictions that cover them, in the free software and open source communities, as well as the broader free culture movement. For example, they are used to distinguish freeware (software gratis) from free software (software libre).
Richard Stallman summarised the difference in a slogan: “Think free as in free speech, not free beer.”
“Free as in beer” is the easiest concept to understand—free beer is a gift given to you at no cost with no expectations of you. The giver simply needs to pay for the beer and give it to you to enjoy without you needing to do anything. This is the “gratis” part of the phrase meaning “at no cost.”
On the other hand, “free as in speech” is a matter of liberty and not just the ability to get the software for free. This liberty (libre) gives you four rights that a free beer does not:
You, as the user, have the right to run the software however you would like. Meaning if you have a computer that runs it, great! If you have a phone or calculator that can run the software too, even better.
You have the right to seeing how the software actually works. This would be akin to knowing the secret ingredients in your favorite beer or soft drink. With free beer, the consumer doesn’t have that freedom.
You are also able to redistribute the software however you’d like. Whether that means you would package the software as part of your own program or simply provide a mirror so your friends can download it directly from you.
You have the right to improve the program, assuming you know how to, and submit those improvements so the public can benefit from your efforts.
― What Does “Free as in Speech” or “Free as in Beer” Really Mean?
See also: wikipedia/en/Beerware