Partitions on Basic Disks
Each disk can have from one to four partitions. All partitions can be primary, or there can be up to three primary partitions and one extended partition, or just a single extended partition. However, a disk cannot have more than one extended partition, and no more than four partitions total.
Primary partitions are assigned drive letters, but extended partitions are not. Instead, logical drives are created within the extended partition, and drive letters are assigned to the logical drives. Drive lettering for hard drives begins at C and can run all the way to Z.