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NKVD troika or Special troika (Russian: особая тройка), in Soviet Union history, were institutional commissions of three persons who issued sentences to people without trial. These commissions were employed as an instrument of extrajudicial punishment introduced to supplement the Soviet legal system with a means for quick execution or imprisonment. It began as an institution of the Cheka, then later became prominent again in the NKVD, when it was used during the Great Purge.
'Troika' literally means "a group of three", "triad", in Russian.
The first "operational troikas" (оперативная тройка) were introduced in the "centre", in the Moscow military okrug in 1929. The qualifier "operational" denotes they were based on the operational departments of the OGPU. Gradually, troikas were introduced to other parts of the Soviet Union for various and different purposes: "court troikas" (судебная тройка), "extraordinary troikas" (чрезвычайная тройка), and "special troikas" (специальная тройка).
Repression of kulaks and others 
A notable step was the NKVD Order no. 00447 by July 30, 1937 О репрессировании бывших кулаков, уголовников и других антисоветских элементов ("About repression of former kulaks, criminals, and other anti-Soviet elements") undersigned by Nikolai Yezhov. By this order, troikas were created on the levels of republic, krai, and oblast. Investigation was to be performed by 'operative groups' "in a speedy and simplified way", the results were to be delivered to troikas for trials.
The chairman of a troika was the chief of the corresponding territorial subdivision of NKVD (People's Commissar of a republican NKVD, etc.) Usually a troika included the prosecutor of the republic/krai/oblast in question; if not, he was allowed to be present at the session of a troika. The third person was usually the Communist Party of the Soviet Union (CPSU) secretary of the corresponding regional level. The staff of these troikas were personally specified in the Order # 00447.
Protocols of a troika session were passed to the corresponding operative group for executions of sentences. Times and places of executions of death sentences were ordered to be held in secret.
Troikas of this purpose functioned for about a year.
Repression of Poles and other ethnic groups 
On September 15, 1939 the Politburo issued the resolution (# П64/22) about the creation of special troikas (Особая тройка) for the period of the Polish operation of the NKVD, which existed about two months. Later the experience was re-used during other national operations of the NKVD: Romanian, Latvian, Finnish, Ukrainian.
The institution of troikas in the USSR was abolished in 1938 by the Decree about Arrests, Prosecutor Supervision and Course of Investigation, issued jointly by the Sovnarkom and Central Committee of the CPSU.