Dux

Dux

Dux is Latin for leader and later for Duke and its variant forms ....

 Britanniarum was a military post in Roman Britain

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

, probably created by Diocletian

Diocletian

Diocletian |latinized]] upon his accession to Diocletian . c. 22 December 244  – 3 December 311), was a Roman Emperor from 284 to 305....

 or Constantine I

Constantine I

Constantine the Great , also known as Constantine I or Saint Constantine, was Roman Emperor from 306 to 337. Well known for being the first Roman emperor to convert to Christianity, Constantine and co-Emperor Licinius issued the Edict of Milan in 313, which proclaimed religious tolerance of all...

 during the late third or early fourth century.

It is listed in the Notitia Dignitatum

Notitia Dignitatum

The Notitia Dignitatum is a unique document of the Roman imperial chanceries. One of the very few surviving documents of Roman government, it details the administrative organisation of the eastern and western empires, listing several thousand offices from the imperial court down to the provincial...

 as being one of the three commands in Britain, along with the Comes Britanniarum

Comes Britanniarum

Comes Britanniarum was a military post in Roman Britain, with command of the mobile field army from the mid 4th century onwards.It is listed in the Notitia Dignitatum as being one of the three commands in Britain, along with the Dux Britanniarum and Comes litoris Saxonici...

and Count of the Saxon Shore

Count of the Saxon Shore

The Count of the Saxon Shore for Britain was the head of the "Saxon Shore" military command of the later Roman Empire.The post was possibly created during the reign of Constantine I and was probably existent by AD 367 when Nectaridus is elliptically referred to as one by Ammianus...

. His troops were limitanei

Limitanei

The limitanei, meaning "the soldiers in frontier districts" The limitanei, meaning "the soldiers in frontier districts" The limitanei, meaning "the soldiers in frontier districts" (from the Latin phrase limes, denoting the military districts of the frontier provinces established in the late third...

or frontier guards and not the comitatenses

Comitatenses

Comitatenses is the Latin plural of comitatensis, originally the adjective derived from comitatus , itself rooting in Comes .However, historically it became the accepted name for...

or field army commanded by the Comes Britanniarum.

Fourteen units in north Britain are listed in the Notitia as being under his command, stationed in either modern Yorkshire

Yorkshire

Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...

, Cumbria

Cumbria

Cumbria , is a non-metropolitan county in North West England. The county and Cumbria County Council, its local authority, came into existence in 1974 after the passage of the Local Government Act 1972. Cumbria's largest settlement and county town is Carlisle. It consists of six districts, and in...

 or Northumberland

Northumberland

Northumberland is the northernmost ceremonial county and a unitary district in North East England. For Eurostat purposes Northumberland is a NUTS 3 region and is one of three boroughs or unitary districts that comprise the "Northumberland and Tyne and Wear" NUTS 2 region...

. Archaeological evidence indicates there were others that were occupied at the time which are not listed. His forces included three cavalry vexillatio

Vexillatio

A vexillatio was a detachment of a Roman legion formed as a temporary task force created by the Roman Army of the Principate. It was named from the standards carried by legionary detachments, vexillum , which bore the emblem and name of the parent legion...

nes with the rest being infantry, overall they were newly-raised units rather than being third century creations. In addition to these fort garrisons, the dux commanded the troops at Hadrian's Wall

Hadrian's Wall

Hadrian's Wall was a defensive fortification in Roman Britain. Begun in AD 122, during the rule of emperor Hadrian, it was the first of two fortifications built across Great Britain, the second being the Antonine Wall, lesser known of the two because its physical remains are less evident today.The...

and the Notitia lists their stations from east to west as well as some further forts on the Cumbrian coast. These troops appear to have been third century regiments although the reliability of the Notitia makes it difficult to infer any solid information from it.