In these situations, there is not copyright, thus there is no copyright infringement. Without going to much in it, the common law that we have is based on precedent. Attorneys and judges are always citing the wise words used by those that have gone before them. The only thing you need to remember to do is to cite where the quotation has come from.
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Not so fast... When a federal judge writes an opinion, this is probably work for hire for the federal government which is not eligible for copyright protection. The state court judges are a different story and the state may very well be entitled to protect its copyrights, as a number of cases have held.
As far as briefs drafted by law firms, this is writing like any other and those law firms are entitled to copyright protection.
Needless to say that in this age of digitization of content, plagiarism gets exposed very easily, so even if you do not get in trouble for copying copying someone else's work, the credibility of your brief would suffer immensely and your judge/jury will not be pleased.
There is a technical copyright law distinction between an opinion and a brief, but in the end not an important one. No suit is going to be successful, as no damages are going to exist.
Copying from an opinion is certainly legal if done for purposes of citing case law. A Federal opinion is exempt from copyright under 17 USC 105 and a state opinion can be copied under 17 USC 107 fair use to use it.
Copying from a brief is another matter, that is infringement under 17 USC 501. However, I have never heard of a brief being registered for copyright with the US Copyright Office, and unless the brief is registered you cannot be sued for copying it, and even if later registered statutory damages will not be available and only actual damages would be possible, which are not going to be present in the case of a brief and certainly not in an amount sufficient to justify a suit.
That said, if you get copying from another brief, the fall out could be substantial, as court and client will seldom find it acceptable.
Bottom line is NO you will not be sued successfully for copying from a brief, but it's not often smart.
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With regard to copying another attorney's brief, it may very well constitute copyright infringement. That will depend on the extent to which the copied content is in fact original to the other attorney, whether it is being copied for fair use purposes (e.g., comment or criticism), and other factors. As other responding attorneys have noted, it is extremely rare for attorneys to register the copyright in their briefs, and rarer still to file suit for copyright infringement. However, such suits have in fact been in the news recently. See http://blogs.wsj.com/law/2012/02/22/keep-your-hands-off-my-briefs-lawyers-sue-westlaw-lexis/. I'm aware of one article from a federal Court of Appeals judge suggesting that such suits could be viable (sorry I can't find the a link to the article).
As for copying language from a judicial opinion, such language is clearly not protected by copyright if it comes from a federal judge. If it comes from a state judge, it is not well-settled whether it is public domain. The statutory prohibition on copyright protection for government works only applies to works of the federal government, and some states do claim copyright protection in government works. See http://www.azlibrary.gov/policies/copyright.aspx. However, one federal appellate court has held that local building codes lose copyright protection when they become "the law," and this rationale would also seem to apply to judicial opinions. See http://www.law.cornell.edu/copyright/cases/293_F3d_791.htm. However, even if a state judicial opinion were protected by copyright, quoting the language would qualify as fair use in many instances.
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