To achieve a band nine in the IELTS test, you have to average at least 8.75 among your four skills. So, for example, if you received band nine for speaking, listening and reading, but band eight for writing, that average would be 8.75, so it would be rounded up to band nine. The similar thing is if you get two band nine scores and two 8.5 scores, it's still possible to get band nine.