Geometry consists of 2 values: heads (per cylinder) and sectors (per track). These values can differ from disk to disk, but are generally 255 (heads) and 63 (sectors). Although modern operating systems don't need to do this anymore, these values are still used when creating (basic) disk partitions. As such it is important that the PC has the correct values for the disk; if the values are wrong the operating system will not be able to find certain important structures on the disk and fail to boot, or even fail to access the partition completely.
