Roman Republic and Roman Empire
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| Varied over unit lifetime. Approx. 3,500 fighting men + support at the time of creation. Expanded and given the cognomenGemina in 31 BC.
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| Hispania Tarraconensis (31 BC - c. 71) Nijmegen (71 - 103) Vienna (103-5th century)
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| Equestris, "Of the knights" under Caesar Gemina, "The twin" (since 31 BC) Pia Fidelis, "faithful and loyal" (since 89) Domitiana, Antoniniana, Gordiana, Deciana, Floriana, Cariniana (short-lived) Pia VI Fidelis VI (after 260)
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| Bull
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! style="text-align: left;" | Notable battles or wars:
| Gallic Wars (58-51 BC) Battle against the Nervians (57 BC) Battle of Gergovia (52 BC) Battle of Ilerda (49 BC) Battle of Dyrrhachium (48 BC) Battle of Pharsalus (48 BC) Battle of Munda (45 BC) Battle of Philippi (42 BC) Battle of Actium (31 BC) Batavian rebellion (70) Bar Kokhba's revolt (132-135) Naissus (268) Vexillationes of the 10th participated in many other campaigns.
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| Julius Caesar, Mark Antony
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Legio X Gemina, the twin legion, was one of the four legions used by Julius Caesar in 58 BC, for his invasion of Gaul. There are still records of the X Gemina in Vienna in the beginning of the 5th century. The legion symbol was a bull. Early on its history, the legion was called X Equestris (mounted), because Caesar once used the legionaries as cavalry.
In the Gallic wars, X Equestris played an important role on Caesar's military success and for this reason is sometimes said to be his favorite. In Caesar's campaigns they were present in the battle against the Nervians, the invasions of Britain, and the siege of Gergovia. They remained faithful to Caesar in the civil war against Pompey, being present in the battles of Pharsalus (49 BC) and Munda (45 BC). In 45 BC Caesar disbanded the legion, giving the veterans farmlands near Narbonne.
Augustus
The legion was reconstituted in 42 BC and fought for Augustus (then Octavian), Lepidus and Mark Antony in the Battle of Philippi against the murderers of Caesar. After this, they followed Mark Antony in his campaign against Parthia and were defeated with him at Actium. Augustus then took control of the legion and settled the veterans in Patras. The legion rebelled and lost its cognomenEquestris as punishment. Reinforcements were added from other legions, and the Tenth was rebaptized Gemina.
As part of the army of Germania Inferior, X Gemina fought against the rebellion of the governor of Germania Superior, L. Antonius Saturninus, against Emperor Domitian. For this reason, the Tenth — as well as the other legions of the army, I Minervia, VI Victrix, and XXII Primigenia — received the title Pia Fidelis Domitiana, "faithful and loyal to Domitian", with the reference to the Emperor dropped at his death.
X Gemina supported its governor, Septimius Severus, in his bid for purple, and many man of the legion went to Rome to become part of the Praetorian Guard of the new Emperor.
3rd century
During the 3rd century, the legion fought for several emperors, who awarded the legion with titles showing the fidelity of the legion and the favour gained by the Emperor himself. The titles Antoniniana (awarded by Caracalla or Elagabalus), Gordiana (by Gordian III), Deciana (by Decius), Floriana (by Florianus), and Cariniana (by Carinus) were short-lived, however, and dropped after the death of the Emperor. For its support of Emperor Gallienus against Postumus, the Gemina was awarded the title Pia VI Fidelis VI, "six times faithful, six times loyal".
4th century
At the time in which Notitia Dignitatum was written (early 4th century), the a first detachment of Decima Gemina was under the command of the Magister Militum per Orientem, and was a comitatensis unit. The other detachment was still in Vindobona, under the command of the DuxPannoniae primae et Norici ripensis.
