During the negotiations I described here, in early 1989, the Polish government agreed to hold an election which, while not being totally free, allowed the first element of multi-party competition in that country, since the establishment of its Marxist-Leninist regime in the wake of World War II.
Election day was set for June 4. The unicameral parliament had been made bicameral, with the addition of a Senate. 35% of the seats in the lower house, and 100% of the Senate seats, could be contested by all parties.
Solidarity, the trade union that had emerged from the Gdansk shipyard strike of 1980, turned itself into a political party, and contested as many seats as were allowed to it. They won their maximum possible total of 35% of the lower-house seats, and 99 of 100 Senate seats.
While the reservation of 65% of the lower-house seats to the Communists was intended to safeguard their power, it didn't work out that way.
The Communist power structure in Poland had been a one-party system in effect, but not in form. Some small parties had been allowed to exist, as long as they did not challenge the Communists' monopoly on power.
The sheer force of 1989's overwhelming anti-Communist vote forced Poland's rulers to allow a coalition government to be formed between Solidarity and those other parties. As a face-saving gesture the Communist leader Wojciech Jaruzelski was given the title of president. But Solidarity's Tadeusz Mazowiecki became prime minister.
As I wrote here, the opening up of the physical Iron Curtain was arguably the straw that broke Marx's back, but the precedent that was set in Poland, that Communists could be voted out of power in a Soviet bloc country, was also very important.