The Triarii (Latin singular triarius) was the third standard line of infantry of the Roman Republic's army. Its name is related to the Latin word tres ("three"), ultimately derived from Proto-Indo-European *trei-. When suffering defeat, the first and second lines, the Hastati and Principes, fell back on the Triarii to attempt to reform the line and allow for a counter attack or withdrawal of the other lines. Because falling back on the Triarii was an act of desperation, to mention "falling on the Triarii" ("ad triarios rediisse") became a common Roman phrase indicating one to be in a desperate situation. To be the officer of the triarii was an honor. It made the individual one of the best men in the legion. A triarii officer's pay would be superior to the other middle-class officers and he would be given a horse for the long marches, which was quite important.Triarii were sometimes, not unlike the Principes, divided into ten maniples of 160 men. Each maniple consisted of two centuries consisting of 80 men each, commanded by a centurion. In other cases they were in fact divided into ten maniples of only 120 men, the size of a century of Hastati or Principes. Hence in many battles the Triarii numbered only half as many as the Hastati or the Principes. The Triarii were the veterans of the Roman army, making them perfectly suited for reinforcing the two front lines of the formation. They were armored much like the rest of the Roman Republican army, with whatever they could afford (typically more than the poorer Hastati and Principes).
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