Another copyright rant, sorry. This one is about copyrightability and plagiarism.
Knitting and crochet patterns aren't copyrightable. They are algorithms (ordered instructions), like recipes, or... algorithms. Algorithms are not copyrightable. HOWEVER, expressions of algorithms can be copyrightable. And also, algorithms can be patented.
I don't think anyone has ever tried patenting a crochet pattern before, thankfully, but it is hypothetically possible. It would likely fall through though since, like cooking, most ways of crocheting or knitting are so extremely ubiquitous that you would have to do something really weird and unique in order for the patent office to grant you something.
The copyrightability of patterns gets into the elements which comprise a work. A knitting pattern is a few things. It's usually photos (copyrightable), detailed explanations of specific joining methods (copyrightable), but also the knitting algorithm which produces the work. The algorithm cannot be copyrighted, and usually the way it's written also cannot be copyrighted. If your algorithm looks like "4k 5p 4k turn" etc. then it's not copyrightable, same if you use "knit four, purl five, knit four, turn" since both are extremely obvious ways to express the algorithm. If you instead wrote a poem to convey the algorithm, that's copyrightable, but the algorithm itself is still uncopyrightable.
You can think of a pattern as having two layers to it. There's the layer that us mortals see, the PDF or the webpage or whatever, and also a Platonic layer which no imperfect creature may fully witness. The Platonic layer is the pure conveyance of the algorithm behind the pattern, it is free of copyright because copyright is an imperfect idea by mortals. The mortal layer is copyrightable. Usually you can even see a glimpse of that perfect Platonic layer in the pattern in the form of notation.
This means that you can right now go to any pattern and take the algorithm, convey it in your own form (or in standard notation) and do whatever you want with it. You of course shouldn't though.
This is where you get into the distinction between copyright and plagiarism. Copyright is a legal doctrine, whereas plagiarism is chiefly a social convention. The Platonic algorithm has no copyright, but in the realm of us mortals we still consider it wrong to take something someone else made and claim it as one's own.
A lot of times copyright (the broad ability to control the copying of a work) and plagiarism (the truthful claim to authorship of a work) line up, but a lot of times they don't. Algorithms, formula, recipes, and other objective instructions are a good example of where they don't align. These are too pure for us mortals to taint with copyright, but since plagiarism is a social convention that we have within our fields, we can still tell someone who rewrites recipes in their own words to fuck off and exile them.
