AM (also written A.M., am, a.m.) is an abbreviation that stands for ante meridiem, which is Latin for "midday." It is written after times that we refer to as morning. For example, we could write 8:10 in the morning or 8:10 AM. Both mean the same time:
Since there are 24 hours in a day, it is necessary to write either AM or PM when referring to the time on a 12-hour clock to indicate the morning or the afternoon. AM refers to the time from midnight (12:00 AM) to noon (12:00 PM). On the 24-hour clock, these would be the times between 00:00 and 12:00.
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The 12-hour clock is much more commonly used in countries that were formerly part of the British Empire. This includes countries such as the United Kingdom, India, Pakistan, Malaysia, Mexico, the Philippines, and more. Some countries use a combination of 12-hour or 24-hour systems, while others exclusively use the 24-hour clock. In general, the 24-hour clock leaves less room for confusion, since you don't have to distinguish between AM and PM.