How do you OCD?

Here are some lovely photos of some well organized shelves. They beg the question: how do you choose to organize your tomes? Alphabetical, by color, by genre, by author, by year....? I tend to save one side of my shelves for yet-to-be-read books & keep my finished ones on the other side & within those two groups I do it by genre or author, which tend to be similar... I also have mild OCD, so shape & size of said books is an important factor to consider.