Politburo
The small committee at the top of a communist party where real power sat, above governments that existed mainly to execute its decisions.
If you want to know who governed the Soviet Union, do not look at the government. The Council of Ministers ran ministries, the Supreme Soviet passed laws unanimously, and neither decided anything. Power sat in a party committee of ten to fifteen men called the Political Bureau, and the single most important fact about the communist state is that its real constitution was the party's rulebook, not the country's.
Origins
The first Politburo was improvised in October 1917 to manage the seizure of power and then lapsed. It was made permanent in March 1919, when the party congress created a body to decide questions too urgent to wait for the much larger Central Committee. Seven members: Lenin, Trotsky, Stalin, Kamenev, and Krestinsky, with two candidates. Alongside it sat the Orgburo, handling organization, and the Secretariat, handling personnel and paperwork.
Everyone assumed the Politburo mattered and the Secretariat was clerical. Stalin took the Secretariat, and in 1922 the new post of General Secretary. Controlling appointments meant controlling who sat in the bodies that elected the Politburo, and within six years the clerical job had eaten all the others. It is the clearest case in modern history of bureaucratic position beating personal charisma.
How it worked
Formally the Politburo was elected by the Central Committee, which was elected by the Party Congress, which was elected by lower party bodies. In practice the flow ran the other way: the Politburo decided who would be nominated, and the nomination was the election. This machinery had a name, the nomenklatura, meaning the schedule of positions the party filled and the approved list of people to fill them. Every job that mattered anywhere in the country, factory director, newspaper editor, general, ambassador, was on that schedule.
The 1934 congress illustrates what the machinery could do to the people who ran it. Delegates arrived to celebrate the achieved socialism of the first Five Year Plan. Of the 1,966 delegates present, well over half were arrested in the terror that followed, and a majority of the Central Committee they elected was shot.
Stalin renamed the body the Presidium in 1952 and enlarged it, a move his colleagues read correctly as preparation for another purge; he died before it happened. It reverted to Politburo in 1966 and kept that name until the Soviet collapse.
The successor
The institution outlived the state that invented it. The Chinese Communist Party runs a Politburo of roughly twenty-four members and, above it, a Standing Committee of seven that is the actual government of China. Xi Jinping's formal title that matters is General Secretary, the same office Stalin used, and for the same reason: it controls appointments. The Party Congress every five years performs the same ratifying function it performed in Moscow.
Why the form persists
The design solves a specific problem. A revolutionary movement that claims to embody the will of a class cannot submit that claim to a vote, because a vote might contradict it. So it builds a body small enough to act decisively, insulated from any electorate, and formally accountable to institutions it appoints itself. That is extremely effective at holding power and extremely bad at succession, which is why nearly every communist state has had a leadership crisis on the death of its leader, and why watching who sits where at a Chinese party plenum remains a real analytical exercise rather than theater.