Read-only memory or ROM can be divided into Mask ROM which is commonly called MROM and  programmable read-only memory or PROM. MROM chips are physically encoded with data by the vendor. This makes it impossible to change the contents of the memory after fabrication and allows for highly reliable memory. On the other hand, a PROM can be programmed after shipping from a vendor. It can be either OTPROM (One-Time Programmable Read-Only Memory) or EPROM (Erasable Programmable Read-Only Memory). EEPROM or Electrically Erasable Programmable Read-Only Memory is an advancement on EPROM that incorporates an electronic erase.  This allows EEPROM to be erased much faster than EPROM. "Double-e prom", "e-squared", or simply "e-prom" are all nicknames referring to EEPROM. EEPROM and Flash Memory can both be programmed and erased electrically. Generally, EEPROM is erased one byte at a time, while Flash is erased in blocks.