I know System ID (SID) is used to uniquely identify a particular database on a system and when using RAC, all instances belonging to the same database must have unique SID's. Logically speaking, combination of instance name and db name can aslo uniquely identify a particular database on a system in a non RAC environment without a need for SID. It seems there is some historical significance to the SID in earlier versions of Oracle. Can someone explain or point me in right direction ?