For 12 in the morning, the use of the abbreviation m (12.00 m) is recommended, because the 24 hours of the day are divided in two parts post meridiem (p.m) and ante meridiem (a.m). From 1, 2… until 11 it is a.m and from 13,14…. until 23:59 it is p.m.
These words come from Latin words meridies (midday), ante (before) and post (after). So, the meaning of a.m is ante meridiem, p.m post meridiem and m meridies.
By: Hot Water Club