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AD
ad is normally written in small capital letters and should be placed before the numerals, as in ad 375 (not 375 ad). The reason for this is that ad is an abbreviation of anno domini, which means ‘in the year of our Lord’, which should logically come before the year. However, when the date is spelled out, it is normal to write the third century ad (not ad the third century). It is not written with full stops after the letters. See also BC.