Annual asymmetry index calculated from hemispheric prominence observations carried out at the Madrid Astronomical Observatory from 1906 to 1957. Dashed horizontal line represents a value of the index equal to zero. Vertical solid lines depict the minima of solar cycle according to the Madrid prominence series and the vertical dashed line is the minima of Solar Cycle 16 according to the sunspot number index. Solar cycle numbers are indicated above the abscissa.