There has been alot of contention over wether the triarii were 'the most experienced' soldiers or just 'too old to fight in the front anymore' soldiers.
In Livy's description of the fourth century legionary organisation (Book 8.8 ... triarios ..., veteranum militem spectatae virtutis ...) the triarii are explicitly mentioned as veteran troops of proven worth. Their officers were correspondingly those with the highest prestige, the legion's chief centurion being that in command of the first maniple of triarii or pili as they were alternatively called. The triarii were composed of the oldest soldiers of the legion. As described by Polybius, Book 6 the division may not have been based on strict age limits and adapted to suit available manpower. They enjoyed some exemptions from duty.
Apart from their dory or hasta carried instead of the hyssos or pilum they did not differ in equipment from the other types of legionary heavy infantry. They are not attested as being better armoured than other soldiers. The notion found in some modern publications that such soldiers would have universally worn mail rather than the kardiophylax or pectorale is not based on the available source material, which indicates that such armour was the distinction of men rated at over ten thousand drachmai. Though as older men they would conceivably have been more wealthy on average than younger legionaries, there are no indications that they would all be clad in mail. The description of Vegetius, Epitoma rei militaris 2.16 ( ...triarii cum scutis catafractis et galeis ocreati cum gladiis semispathiis plumbatis binis missibilibus... '... thirdliners with shields, body armour and helmets, greaves, swords and daggers, leadweighted darts and two throwing spears ...') includes vocabulary that suggests contemporary practice rather than copying from earlier sources. As reserves triarii usually kneeled or sat down in anticipation of their use in battle, either to lead a final assault attempt or to cover a retreat. This was not just due to any lesser fitness of the veterans, but usual practice to limit fatigue among reserve troops, a concern recorded in several works on military matters from Antiquity.
Extant evidence indicates that in the imperial army the alternative designation of such troops, pili, was almost universally employed (the restoration of the Caelius stone as listing a t(riarius) ordo is not widely accepted, though the Aurelius Gaius text is definite). The age division seems to have been abandoned, though the pili continued to be the more prestigious of the centurionate. In the first century of the principate at least older soldiers were brigaded together in a vexillum veteranorum, under a curator veteranorum, though due to the absence of continued references it is thought that this practice may have been discontinued later on. There are no clear indications available that these were integrated into the expanded first cohort that some legions appear to have adopted in the later first century AD.
According to Tacitus Annales 3,21 ( ...vexillum veteranorum, non amplius quingenti numero... 'a task force of veterans, not more than fivehundred in number...') there could have been about five hundred veterans in a vexillum veteranorum.
Some, not all, texts mentioning officials associated with such veteran groups.
CIL 5, 4903
L(ucius) Salvius | C(aii) f(ilius) Fab(ia) | vexillarius | veter(anorum) leg(ionis) IIII sibi | et Popiliai(!) | T(iti) l(ibertae) | Hispan[a]i et Capito | ni f(ilio) et Priscai f(iliae) et
| Firmo f(ilio)
'Lucius Salvius, son of Caius, from the Fabian voting district, flagbearer of the veterans of the legio IIII for himself and Popilia Hispana, freedwomen of Titus, and his son Capito, his daughter Prisca and his son Firmus'
AE 1969/70, 133
] Salvius L(ucii) f(ilius) | Pol(lia) Celer vet(eranus) leg(ionis) IIII Scy(thicae) curator | adl(ectus) veter(anorum) aed(ilis) IIvir q(uinquennalis) | mil(itavit) eques an(nos) XXX vix(it) an(nos) LV | hic s(itus) est | Blassia Felicula uxor | | Antonia Fortunata A(ulus) Titinius Italus
'... Salvius Celer, son of Lucius, from the Pollian voting district, veteran of legio IIII Scythica, selected curator of the veterans, aedilis, member of the commission of two, quinqennalis has served as a cavalryman for 30 years. He has lived 55 years. He is buried here. Blassia Felicula, his wife, Antonia Fortunata, Aulus Titinius Italus'
CIL 5, 3375
L(ucius) Sertorius L(ucii) f(ilius) | Pob(lilia) Firmus | signif(er) aquil(ifer) leg(ionis) XI | Claud(iae) Piae Fidelis | missus curat(or) veter(anorum) | leg(ionis) eiusdem | Domitiae L(ucii) f(iliae) | Priscae uxori
'Lucius Sertorius Firmus, son of Lucius, from the Poblilian voting district, standardbearer, eaglebearer of legio XI Claudia pia fidelis, retired, curator of the veterans of the same legion for Domitia Prisca, daughter of Lucius, his wife'
As for the question of formations, Vegetius is not the clearest source for the formation of the Roman Army. The trouble with him is that he picks fragments of earlier and recent practice, weapons and troop-types and mixes them together into a store he likes. For instance: the names of the soldiers in the line (Principes, hastati, triarii and light-armed or velites), he seems to have read in Polybius VI.21-24, but cohorts didn't exist then. Then again, plumbatae are (for him) very recent.
In the Polybian system, the velites would hold a skirmishing line in front of the legion and would withdraw when attacked by heavy infantry. The first line of heavy infantry was formed by the centuries of hastati, the second by the principes (maybe Vegetius was confused by the terminology) and the last line by the triarii. This difference disappears in the Imperial legions, when the formation would be determined by the seniority of the cohort (which is a very late Republican innovation). Vegetius describes this too: 1st cohort right flank, first line, the second next to it etc.
But exactly how the legion would be formed depended on the situation, lay of the land, disposition of the enemy, etc. The legion could be formed up in two lines of five cohorts, three in 4-3-3 etc.
The number of ranks and files would again have been flexible. Within the basic legionary units, century and maniple/cohort, it would be up to the commanding officer to decide about with and depth of the legion's frontage. On a wide front a century could be formed in 10 lines of 8 man, or deeper: 8 and 10. But of course many more options were possible.
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