 Digital Equipment Corporation (DEC) was the first to implement this, avoiding I (1), O (0) and S (5).The remaining letters are called the DEC alphabet.Occasionally, aircraft with a seating structure of 2+2 may letter the seats as "ACDF" to keep with the standard of A/F being window and C/D being aisle on short-haul aircraft (which generally have 3+3 seats).