AD and BC. AD is traditionally written before a number signifying a year: AD 1625. This is because the abbreviation AD means Anno Domini (in the year of our Lord), and if the phrase were written or spoken in full it would make better sense before the date than after it. This practice is by no means universally adhered to with year numbers, however, and wWhen referring to centuries, however, it is necessary to place AD at the end of the phrase: in the third century AD. BC always follows a date: 440 BC; the seventh century BC. In printed text these abbreviations are often written in small capitals: ad and bc