In many settings, open source licensing today is considered a solved problem. Not only has the Open Source Initiative (OSI) largely contained the long feared issue of license proliferation, the industry has essentially consolidated around a few reasonably well understood models.
Copyleft licenses such as the GPL, which require users who would distribute the software to demonstrate reciprocity by making available their changes under the same license (hence the usage of reciprocal to refer to these licenses) exist at one end of the spectrum. So-called permissive licenses, which include the Apache, BSD and MIT licenses, and generally ask very little of users of the code, are at the opposite end. In between are MPL-style licenses, which more selectively apply copyleft-style reciprocity requirements.
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