Within the elect oral arena, the PS, PSD and PP-CDS have increased their centrality over the period covered, although the par ties receive a rel at ively low combined share of the vote. 1985 marks a turning point in the centrality of the estab lished par ties. Before 1985, the three par ties together received around 75 per cent of the vote. In 1985, this value fell to 60.6 per cent as the PRD received 17.9 per cent of the vote. However, this elect oral shock was to be short-lived and since 1987, the three par ties have increased their elect oral centrality to a high in 1995 of 87 per cent. There has been a trend for the centrality of the estab lished par ties within the elect oral arena to increase over the period covered. Ironically, it appears that the collapse of elect oral centrality in 1985 acted as a cata lyst for the estab lished par ties to increase their centrality to the present high levels. The parlia ment ary arena reflects the elect oral arena rel at ively closely due to Portugal’s proportional elect oral formula. The data shows that 1985 again is the low point of estab lished party centrality within the parlia ment ary arena, with the three estab lished par ties receiving a combined share of only 66.8 per cent of seats. Previously, the par ties had received around 83 per cent, but after 1985, their parlia ment ary centrality increased dramatically. The major trend in the parlia ment ary arena is sim ilar to that found in the elect oral arena. Before 1985, Portuguese estab lished par ties dem on strated low levels of sys temic centrality and the shock of 1985 actu ally served to increase their centrality, as over the past two decades, Portuguese estab lished par ties have maintained high levels of parlia ment ary centrality. Levels of gov ern mental centrality are sim ilarly high. One of the most interesting features of the Portuguese data is the mono poly of gov ern ment by the PS, PSD and PP/CDS. This dem on strates high levels of centrality, as the estab lished par ties have been able to maintain their mono poly on governing positions and have not allowed new par ties to enter gov ern ment. However, inde pend ent min isters have an im port ant role to play in gov ern ment formation in Portugal, so although only the three estab lished par ties have parti cip ated in gov ern ment, inde pend ent min is ters have often proved to be im port ant gov ern mental actors. Indeed, the second gov ern ment of 1978 was made up entirely of inde pend ent min is ters under the control of the pres id ent (Magone 2000: 238), but more recently, 47.1 per cent and 41.2 per cent of min is ters in the gov ern ments formed in 2005 and 2009 respectively have no party af fili ation. Within the gov ern mental arena, a clear trend is for the mono poly of party governing positions by the three estab lished par ties, but not a mono poly on gov ern ment as a whole. Having con sidered the centrality of the estab lished par ties within the Portuguese party sys tem, it is also im port ant to con sider whether any changes have occurred in the party sys tem as a whole. The study has found increasing levels of centrality of estab lished par ties within the party sys tem, with levels of centrality in all three arenas rel at ively high, and wider party sys tem trends should similarly reflect estab lished party dominance.