Time subdivisions

.AA0	Prehistoric (-3500 BC) Prehistoric means before the dawn of the great civilisations of the Near and Far East, ca. 3500 BC (technically before the invention of writing). Civilisation did not arise till later in Europe and the Americas, so the title and content should dictate the cataloguer’s choice of term.
.AA1	Ancient period (3500BC-500 AD)
.AA2	Middle Ages (500-1300)
.AA3	14th century
.AA4	15th century
.AA5	16th century
.AA6	17th century
.AA7	18th century
.AA8	19th century
.AA9	20th century
.AA10	21st century

There may be a span of dates, eg .AA5-7, but do not leave an open span. Rather than doing so, do not use a period subdivision at all.