There is no line that defines what's copyrightable and what's not. That's why we have courts to decide the borderline stuff. The 80:20 ratio of software develo

There is no line that defines what's copyrightable and what's not. That's why we have courts to decide the borderline stuff. The 80:20 ratio of software development applies to the legal system as well. Tightly specifying the legal code for the borderline 20% would require so much work and added complexity to be almost impossible. Hence we instead use the court system to decide on the borderline cases. Generally a wide range of factors are taken into account when quoting a work. This includes the length of the quotation (in proportion to the whole work) and the context the quotation is used in. So for example short poems (<10 words) have received protection in the past, because the 10 word quotation forms a substantial proportion of the work. The argument for protecting headlines comes down to whether headlines are works in their own right. Given that they can stand on their own independent of the articles they're associated with means that the probably do qualify to be treated as works on their own right. The creativity and originality criteria are tricky, it's likely that some headlines are copyrightable but others aren't. Only a court judgement can clearly but a headline in one category or other. But as I said elsewhere, this is only relevant if you are copying from another source. If you write a headline that happens to be identical to another headline then that isn't a copyright violation (regardless of length).