<P> A decade is a period of 10 years . The word is derived (via French and Latin) from the Ancient Greek: δεκάς, translit . dekas), which means a group of ten . Other words for spans of years also come from Latin: biennium (2 years), triennium (3 years), quadrennium (4 years), lustrum (5 years), century (100 years), millennium (1000 years). </P> <P> Although any period of ten years is a decade, a convenient and frequently referenced interval is based on the tens digit of a calendar year, as in using "1960s" to represent the decade from 1960 to 1969 . Often, for brevity, only the tens part is mentioned (60s or sixties), although this may leave it uncertain which century is meant . These references are frequently used to encapsulate popular culture or other widespread phenomena that dominated such a decade, as in The Great Depression of the 1930s or "The Roaring Twenties". </P> <P> However, the Gregorian calendar begins with the year 1 . (There is no year "zero", and the year before AD 1 is 1 BC with nothing in between .) Therefore, its first decade is from AD 1 to AD 10, the second decade from AD 11 to AD 20, and so on . So, although "the 1960s" comprises the years 1960 to 1969, "the 197th decade" spans 1961 to 1970 . </P>

What do you call a period of 20 years