According to Annals of Tacitus there are three words that derive from "Arsac":

  • Arsacis (e.g. VI:33)
  • Arsacidis (XIII:37)
  • Arsacidarum (e.g. XII:10)

What are the differences between these three words in Latin grammar?

1 Answer 1


Tacitus gets his names from Greek:

  • Greek Ἀρσάκης became Latin Arsacēs, i.e. Arsaces, the Parthian king,
  • Greek Ἀρσακίδης became Latin Arsacida, i.e. Arsacid, one of his dynastic successors.

Arsacis is a genitive singular of the first word: "of Arsaces". This treats the word as if it's in the third declension with a stem Arsac-, which is unusual (the Greek is in the first declension) but not extremely so.

Arsacidis is a dative plural (formally it could also be an ablative plural, but in this passage it's a dative) of the second: "to the Arsacids". Arsacidarum is a genitive plural of the same: "of the Arsacids". Here Tacitus treats the word as being in the first declension, just as the Greek is, even though masculine first-declension words are comparatively very rare in Latin.

The reason the first word gets put in the third declension is because personal names tend not to get changed more than is necessary, which leads to awkward fits in the grammatical system, and unless a noun is formally a very obvious match for the first or second declension (i.e. the nominative ends in -a, -us, or -um) it defaults to the third. (If Ἀρσάκης were adapted to the first declension it would be Arsaca.)
The second word does get adapted to the Latin first declension because people did realise the Greek first declension was equivalent to the Latin first declension in some way, and as it's not a person's name, more leeway was afforded to change it.

  • 1
    In Tacitus VI, 33, Arsaces happens not to be “the Parthian king,” but a son of latter-day Parthian king Artabanus, who was made king of Armenia by his father. Also note that the nom.sg. of Arsacidae seems to be Arsacides, although that's not found in Tacitus (but the acc. Arsaciden is, which has apparently escaped the keen eye of the OP). May 6, 2023 at 15:19
  • Linguistically, is it possible "genus Arsacis" (Arsaces) and "gens Arsacidarum" (Arsacid) refer to two different houses within the Arsacid dynasty? (see: Olbrycht, Marek. 2012. The Political-Military Strategy of Artabanos/Ardawān II in AD 34-37, p220)
    – user64617
    May 6, 2023 at 15:25
  • 1
    @user64617 “Linguistically” these are different things. See how the two terms in Tac. VI, 33 are rendered in the Loeb translation by J. Jackson: “as they were unable to bestow the crown on a scion of the Arsacidae” (neminem gentis Arsacidarum); “a descendant of Arsaces should be seen upon the bank of Euphrates” (genus Arsacis). Of course in this context these two expressions mean essentially the same thing, not two different houses. May 6, 2023 at 16:20

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