The MPL Reciprocity Bargain

Although the MPL is a much longer license than the others I've discussed, when you get beyond the complex words, its reciprocity provisions can be paraphrased very simply as follows : If you create and distribute a Modification to one of the files containing Original Code or previous Modifications, or create and distribute a new file containing Original Code or previous Modifications, those files must be released as Modifications under the same MPL license.

Your newly released files become Modifications for future licensees . One can recognize in this recursive definition how a chain of title is created to ever-more-modified derivative works, with each Contributor adding to the chain. But here, unlike with previous licenses, the MPL deals with files containing derivative works rather than derivative works more broadly.

This calls for precise definitions, which the MPL provides. Here are four of them:

"Contributor" means each entity that creates or contributes to the creation of Modifications. ( MPL section 1.1.)

"Covered Code" means the Original Code or Modifications or the combination of the Original Code and Modifications, in each case including portions thereof. ( MPL section 1.3.)

"Modifications" means any addition to or deletion from the substance or structure of either the Original Code or any previous Modifications. When Covered Code is released as a series of files, a Modification is: A. Any addition to or deletion from the contents of a file containing Original Code or previous Modifications. B. Any new file that contains any part of the Original Code or previous Modifications. ( MPL section 1.9.)

"Original Code" means Source Code of computer software code which is described in the Source Code notice required by Exhibit A as Original Code, and which, at the time of its release under this License is not already Covered Code governed by this License. ( MPL section 1.10.)

Such definitions are extremely important in software licenses. You have already seen how words in simple academic licenses, and even some words in the venerable GPL, are confusing and subject to misinterpretation. Words without definition are ambiguous; there is no reliable way to predict how the parties ”or a court ”might interpret a license without clear definitions when performance under it is called for or questioned.

One way to deal with that problem is to rely on terms of art , words that will be understood by the parties and by courts because they are defined in the legal lexicon or by statute . That is why I have been so adamant in this book about using the terms collective works and derivative works precisely. Those terms are defined by statute for all lawyers to understand (although few really do), and if we use them consistently we'll at least all mean the same thing. We can also use court decisions from similar cases to help us predict how the courts will interpret certain terms of art in our own licenses.

Commercial open source licenses like the MPL, in addition to using legal terms of art precisely, often rely on their own definitions of terms. (The GPL did that for the term Program .) Those definitions must be read carefully because, in license interpretation and enforcement, they often take precedence over the terms of art . For example, the four definitions I quoted from the MPL above distinguish carefully between Covered Code and Original Code ; the latter is included in the former. Note that the term Modifications is defined in light of Original Code in its first sentence and Covered Code in its second sentence. We must parse very carefully to know our reciprocity obligations under such licenses.