The 24-hour day is divided into two sections by the 12-hour clock. The term 'am' is derived from the Latin ante meridiem, which literally means 'before noon.' When the sun has not yet crossed the meridian, this is the case. Post meridiem is an abbreviation for 'after midday,' which occurs after the sun crosses the meridian. Ante meridiem is commonly denoted as AM, am, a.m., or A.M. and post meridiem is usually abbreviated PM, pm, p.m., or P.M.