In Roman city planning, a decumanus was an east-west-oriented road in a Roman city, castra (military camp), or colonia. The main decumanus was the Decumanus Maximus, which normally connected the Porta Praetoria (in a military camp, closest to the enemy) to the Porta Decumana (away from the enemy).

This name comes from the fact that the via decumana or decimana (the tenth) separated the Tenth Cohort from the Ninth in the legionary encampment, in the same way as the via quintana separated the Fifth Cohort from the Sixth.

In the middle, or groma, the Decumanus Maximus crosses the perpendicular Cardo or Cardus Maximus, the primary north-south road that was the usual main street. The Forum is normally located close to this intersection of the Decumanus Maximus and the Cardus Maximus.

Examples

In the ancient Roman city of Barcino (present day Barcelona, Spain), the Decimus Maximus started at the Roman gate Placa Nova, which is the only Roman gate extant. In present day Croatia within the city of Split is the UNESCO Roman monument, Diocletian's Palace. This city built by Emperor Diocletian exhibits the characteristic Roman orthogonal street system with the Decumanus Maximus connecting the west Silver Gate to the east Iron Gate. At the present day city of Umm Qais, Jordan, the decumanus is extant running east-west for approximately one kilometre with its ancient flagstones extant; the name of this Roman city is Gadara.

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