Tapes are primarily used for backing up computer systems for resilience in case of failure. They consist of a spool (or spools) of long magnetic tape that can be used to sequentially write data to. Due to the nature of being sequential, they don't lend themselves well for updating frequently, for example, during the time a program is running and constantly needed to read and write data to the tape. Hard discs are a far better device for this purpose. However, tapes can be long and very thin, allowing a very high density of data per inch of tape to be stored. This means that full computer systems can be "spooled" to tape. If the computer then fails, this data and program information can be restored after the repair has taken place. Storage device sizes are measured in bytes, a byte representing a single character of information. See also Storage Devices, Hard Discs, Kilobyte, Megabyte and Gigabyte.