<P> A decade may also refer to an arbitrary span of ten years . For example, the statement "during his last decade, Mozart explored chromatic harmony to a degree rare at the time", merely refers to the last ten years of Mozart's life without regard to which calendar years are encompassed . </P> <P> Particularly for decades of the 20th century, a nominal decade is often used to refer not just to a set of ten years but rather to an era roughly approximating those ten years - for example, the phrase the sixties often refers to events that took place between c. 1964 and 1972, and to memories of the counterculture, flower power, protests of 1968 and other things happening at the time . This practice however is occasionally used for decades of earlier centuries, for example references to the 1890s as the Gay Nineties or Naughty Nineties . </P>

If 10 years is a decade what is 20