In addition to the *excellent* (and accepted) answer posted by Kevin, I want to point out the following: It sometimes argued that having license behaving predictable in a court of law only matters if you want to *restrict* somebody. With the possible exception of a disclaimer of warranty (which may or may not be legal, depending on jurisdiction), there is no *need* to be too fussy about loose, permissive licenses such as [WTFPL](http://www.wtfpl.net/) or the [Unlicense](http://unlicense.org/). If you just want to *be nice* and impose no restrictions on your users, *then* the choice of license does not matter. But it does: - Some managers of free software projects take licensing issues seriously. Something that to non-professionals [looks like a lax and permissive license](http://copyfree.org/content/standard/licenses/coil/license.txt), may not [be what it appears](http://opensource.stackexchange.com/a/1455/606). Such managers may refuse to accept any contribution or pull request unless the code is licensed under terms that are recognized as *compatible* with project's main license. Most "crayon" licenses fail that test. - The public domain be great for cultural works (at least in jurisdictions that recognize the public domain). IMHO, the public domain, or something resembling it is not a good idea for *software* (or anything that is functional, rather than aesthetical). One reason the PD is dangerous for soft5ware is a nasty thing called "software patents". If you put your software in the public domain, somebody else may just go ahead and patent it, and then accuse *your software* of infringing *their* patent. Too far fetched? Well, this is what happened to a programmer named Bob Jacobsen<sup>1</sup>, so it certainly cannot be ruled out. --- <sup>1) Kamind Associates, Inc. (a commercial company) sold a product named "Decoder Commander" that would "normalize" the interfaces for various types of model railway systems as part of a larger framework to let a personal computer control such a system. Katzer, the owner of that company, held various patents related to this software. At one point, Katzer sued a programmer named Bob Jacobsen for infringing those patents. The allegedly infringing software was a set of files know as "DecoderPro", that Jacobsen had made publicly available under a very permissive license (Artistic License 1.0). As it turned out, Katzer had *copied* those files from "DecoderPro", and modified them slightly to create "Decoder Commander".</sup> <sup>In the end, Jacobsen prevailed in court, and the court decided that *Katzer* had infringed on Jacobsen's software. But his poor choice of license (Artistic License 1.0) caused him a great deal of grief.</sup>