I'm working on a new project that'll involve writing my own CMS. It'll be very light weight, and doesn't need to be generic. I'm going over how I'll layout the database tables now and I've been using WordPress's table structure as a guide. I'm not really sure what the purpose of their wp_post2cat table is though. It consists of 3 columns. Relevant_ID, Post_ID, Cat_ID. Ok, so the table holds a list of post IDs and ties in which posts are in which category. That's great. But why? They already store which category the post resides in the wp_posts table (along with other information about the post). They even store the amount of posts in a category inside the wp_category table, so it's not like they are using this table to get a quick count of how many posts are in a specific category. This leads me to my question. Why does this table exist?