How does one know when one has created a new genre of anything? This is not as easy a question to answer as one may think. To be sure, there are genres of literature and music that have been deliberately created, but there are a great many more that can only be recognized after the fact. I have written recently  on the creation of the genre of the sermonette, noting that it was a deliberate creation as a genre to provide more men with the opportunity to speak in services. Similarly, the split sermon was a deliberate creation to divide the whole speaking time between two messages of roughly equal length. In this case the deliberate creation of a genre of homiletical messages was done by viewing an existing genre of religious speaking and writing and dividing it up based on the dimension of time. In a similar fashion length is what separates a novella from a novel–once enough shorter novels were being made it made sense to have a specific name for them.
And that suggests to us that many genres are not necessarily a matter of conscious and deliberate creation, but rather a post facto recognition of a category of works after enough of them have been created to distinguish them on some basis. Did Matthew, Mark, Luke, and John know that they were creating a whole knew genre of writing when they wrote their histories? Did the original author of the last thirteen chapters of Genesis or Ruth know that they were creating early novel-like nonfiction writings that would eventually become part of the history of much larger and later works? It seems quite clear that these writers told their story truthfully and skillfully and then later on what they had done was recognized as a new genre of writing. And this seems to be what has happened in many cases. People had a story to tell and they told it and after the fact someone recognized that the story they told was somehow different than what had come before it and was worth putting in a category all of its own.
There are plenty of genres, though, that are created through rules. For example, a sonnet has fourteen lines of poetry in rhymed iambic pentameter. There are two known types of sonnets, namely the Shakespearean sonnet of four quatrains with a rhyming couplet at the end and the Petrarchian sonnet of an octet and a sextet. Theoretically speaking, someone could create a new sonnet form with seven rhymed couplets, or two septets or two sextets and a rhyming couplet, and it could theoretically create a new genre of sonnet, at least theoretically speaking. Similarly, it would be possible to conceive of a new short poem like the haiku (three lines of poetry with 5-7-5 syllables) or the tanka (five lines of poetry with 5-7-5-7-7 syllables), and the creation of a rule and the presence of enough exemplars to show the viability of the genre could lead to a new genre of that sort of poem as well. The development of new rules can therefore lead to new genres by intention rather than by retrospective analysis.
And that brings us full circle again. We noted at the beginning how the sermonette and split sermon were deliberate genre creations, with a clear point, and have now commented that new poetic genres (and this would apply to music as well) can be formed deliberately through the invention of rules that establish genres. We might therefore say therefore that genre creation occurs via either a top-down or a bottom-up process. A bottom-up process allows people to create what they are creating, and then figures out after the fact various boundary lines that separate one work of art from another, the way that a baroque piece is distinguished from a classical piece, for example. A top-down process, though, formulates rules and then deliberately sets out to populate the genre based on that new rule–for example, the atonal use of twelve-tone music as opposed to the major and minor keys of the Western music tradition. Various people will, of course, prefer one or the other way of creating new genres either after the fact by recognizing distinctions or at the time by formulating new rules in order to generate novelty, but however you prefer to do it, there is no need to despair that everything has been created already, since human beings do not have nearly as good of imaginations about the space of what is possible as we think we do. That is, however, a subject for another time.
 See, for example: