How does the reference system of the Majjhima Nikaya work?

I have come across a reference to the Majjhima Nikaya (MN) noted as MN 1.436. In my own book (The Middle Length Discourses of the Buddha: A translation of the Majjhima Nikaya by Bhikkhu Nanamoli and Bikkhu Bodhi) there are 152 suttas and each sutta can be sub-divided into a number of vaggas but I can't figure out how one arrives at the aforementioned reference (just newly started to take an interest in the literature). Kind Regards Bo