* A maximum of four partitions can be placed on any hard disk. These are sometimes called primary partitions. The limitation of four is one that is imposed on the system by the way that the master boot record is structured.
* Only one partition may be designated, at any given time, as active. That partition will be used for booting the system. See here for more on active partitions and switching active status between partitions.
* DOS (and the operating systems that depend on it for booting, which includes all consumer Windows operating systems) will only recognize the active primary partition. Any other primary partitions will be ignored.
* One of the four partitions may be designated as an extended DOS partition. This partition may then be subdivided into multiple logical partitions. This is the way that two or more logical DOS volumes can be placed on a single hard disk.