and you shouldn't be using any of those, since the order can and will change. The numbers are based on the order the devices and device drivers are initialized in, not based on physical location in the system. The modern approach (assuming you're using udev) is to use the symlinks in /dev/disk/by-id/ or /dev/disk/by-uuid/ instead, since both are consistent across reboots (and by-id should be consistent across reinstalls, assuming the same partitioning scheme on the same physical drives)
This is also why Ethernet devices now have names like enp0s3 - the numbers are based on physical location on the bus. The old eth0, eth1, etc. could swap positions between Linux upgrades (or even between reboots) since they were also just the order the drivers were initialized in.