Traditionally, the disklabel was the first sector of the disk. However, this system only works when the only operating systems that access the disk are Unix systems that comprehend disklabels. In the world of IBM PC compatibles, disks are usually partitioned using the PC BIOS's master boot record (MBR) Partition Table scheme instead, and the BSD partitioning scheme is nested within a single, primary, MBR partition (just as the "extended" partitioning scheme is nested within a single primary partition with extended boot records). Sometimes (particularly in FreeBSD), the primary MBR partitions are referred to as "slices" and the subdivisions of a primary MBR partition (for the nested BSD partitioning scheme) that are described by its disklabel are called "partitions". The BSD disklabel is contained within the volume boot record of its primary MBR partition.