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a.m., p.m.

Lowercase, periods, no space.

Use figures with these terms:
            3 a.m.                         4 p.m.
but     three o'clock           four o'clock
            twelve o'clock
            the 4 a.m. bell (no hyphen)
            the 4 p.m. bell (no hyphen)

Write noon or midnight, not 12 a.m. or 12 p.m.

See also midnight, noon, TIME OF DAY.