I am new to stack overflow. So please correct me if my question seems irrelevant or stupid. I read here in Booting Process : The job of the primary boot loader is to find and load the secondary boot loader (stage 2). It does this by looking through the partition table for an active partition. When it finds an active partition, it scans the remaining partitions in the table to ensure that they're all inactive. When this is verified, the active partition's boot record is read from the device into RAM and executed. The question is that I am having a Hard disk which has two Operating System images windows and ubuntu and hence both partitions in which they reside are active. Then why do we have only one active partition always? (I know that active partition is one of the primary partition but then why we are giving special reference to one primary partition? ) I am confused a bit. Please solve my query. Thank you so much.