- Diocletian created the Tetrarchy (284-305 CE) with 4 Praetorian Prefectures responsible for 12 Dioceses.
1. PRAETORIAN PREFECTURE OF GALLIAE
DIOCESE OF GALLIA
-
- Gallia Lugdunensis I
- Gallia Lugdunensis II
- Gallia Lugdunensis III
- Gallia Lugdunensis IV
-
- Belgica I
- Gallia Belgica II
-
- Germania I
- Germania ii
-
- Alpes Poeninae et Graiae
- Maxima Sequanorum
DIOCESE OF VIENNENSIS
-
- Viennensis
- Alpes Maritimae
- Aquitanica I
- Aquitanica II
- Novempopulana
- Narbonensis I
8Narbonensis II
DIOCESE OF HISPANIAE
-
- Baetica
- Baleares (Balearic Islands)
- Carthaginiensis
- arraconensis
- allaecia
- Lusitania
- Mauretania
- Tingitana or Hispania Nova
DIOCESE OF BRITANNIAE
-
- Maxima Caesariensis
- Valentia
- Britannia Prima
- Britannia Secunda
- Flavia Caesariensis
2. PRAETORIAN PREFECTURE OF ITALY AND AFRICA
DIOCESE OF ITALIA SUBURBICARIA
-
- Campania
- Tuscania et Umbria
- Picenum Suburbicarium
- Apulia et Calabria
- Bruttia et Lucania
- Samnium
- Valeria
- Corsica
- Sicilia
- Sardinia
DIOCESE OF ITALIA ANNONARIA
-
- Venetia and Istria
- Aemilia
- Liguria
- Flaminia and Picenum Annonarium
- Alpes Cottiae
- Raetia I
- Raetia II
DIOCESE OF AFRICA
-
- Africa proconsularis or Zeugitana
- Byzacena
- Mauretania Caesariensis
- Sitifensis
- Numidia
- Triploitana
3. PRAETORIAN PREFECTURE OF ILLYRICUM
DIOCESE OF PANNONIA
-
- Dalmatia
- Noricum Mediterraneum
- Noricum Ripensis
- Pannonia Prima
- Pannonia Secunda
- Savia
- Valeria Ripensis
DIOCESE OF DACIA
-
- Dacia Mediterranea
- Moesia I
- Praevalitana
- Dardania
- Dacia Ripensis
DIOCESE OF MACEDONIA
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- Macedonia Prima
- Macedonia Salutaris (or Macedonia Secunda)
- Thessalia
- Epirus Vetus
- Epirus Nova
- Achaea
- Creta
4. PRAETORIAN PREFECTURE OF ORIENS
DIOCESE OF THRACE
-
- Europa
- Thracia
- Haemimontus
- Rhodope
- Moesia II
- Scythia
DIOCESE OF ASIANA
-
- Asia
- Hellespontus (near Sea of Marmara)
- Pamphylia
- Caria
- Lydia
- Lycia
- Lycaonia
- Pisidia
- Phrygia Pacatiana
- Phrygia Salutaris
DIOCESE OF PONTUS
-
- Bythinia
- Galatia
- Galatia Salutaris
- Phaphlagonia
- Honorias
- Cappadocia I
- CappadociaII
- Helenopontus
- Pontus Polemoniacus
- Armenia I
- Armenia II
DIOCESE OF ORIENS
-
- Arabia
- Palestina I
- Palestina II
- Palestina Salutaris
- Syria I
- Syria II
- Phoenice
- Phoenice Libanensis
- Euphratensis
- Osroene
- Mesopotamia
-
- Cilicia I
- Cilicia II
- Isauria
- Corduene
- Cyprus
DIOCESE OF AEGYPTUS
-
- Aegyptus
- Augustamnica
- Thebais
- Arcadia
- Libya Superior
- Libya Inferior