Noble Titles in Mystara
by Giampaolo AgostaBaron is used in Thyatis, Karameikos, and Glantri, as well as in the Savage Coast, particularly in the Savage Baronies, but also Robrenn, Eusdria, and Renardy (but not Bellayne or Herath, IIRC). Thus, it is IMO clearly a title Thyatian title, which expanded from there to Glantri and the Savage Coast, as well as to Alphatia, albeit in a different sense (an Alphatian Baron is a King who styles himself as a Baron to mark himself as a dashing frontier ruler).
In RW, Baron is of germanic origin, and can be therefore assumed to have originally been a Hattian title for a lesser noble, not unlike the Count (q.v.).
Then, Viscount is not used in Thyatis or Karameikos (note that while there are no Counts in Karameikos, it is assumed by the Gaz that some will be created in the near future), but is used in Glantri and in some nations along the Savage Coast. Due to the clear derivation from Count, my assumption is that it originated in Glantri as Vice-Count, where "Count" was the title of the original leader of the Thyatian settlers.
Count is another title clearly connected with Thyatis, as per the above. It is worth noting that in RW Count is of Latin origin, from comes (companion, meaning a member of the entourage of a ruler). Thus, it seems appropriate that it originated in Thyatis. It is also found along the Savage Coast (Cimarron, Herath, Robrenn, Eusdria, either directly or as part of Viscount), and Glantri, and hypothetically in Karameikos.
IIRC, also the title of Marquis is not present in Thyatis, and therefore can't be derived from Alphatia through Thyatis itself. Marquisates are relatively common in Glantri, and are present in Renardy, which is directly influenced by Glantrian (specifically Averoignese) culture (Lupins first appear as followers of Claude d'Ambreville in X2). Note that in RW, Marquis derives from Markgraf (Count of the Mark, or Borderland). My hypothesis is that Marquis is a Glantrian translation of the Darokinian term "Borderland Magistrate", and was originally associated to dominions close to the dangerous Broken Lands and Ethengar borders.
Note that the title of Margrave (same origin) is used in the Traladaran City States, and that many regions in Ostland, Vestland and Soderfjord are known as "mark", although the local rulers use the "Clan head" style.
Duke is a title that is strongly related to Thyatis (the first Dukes were the Duke of Thyatis, Kerendas and Hattias, the three Thyatian tribes). Archduke is clearly a derivative assigned to the demoted Kings of Ochalea and Pearl Islands. It is worth noting that in RW this is another title of Latin origin, from dux, or leader, a title typically assigned to military leaders who were not true kings (especially barbarian tribe chiefs). In Mystara, it is used in Glantri and in the Savage Coast. In Glantri, it is likely to have originated as the Thyatian translation of the title held by the leaders of the Vlaems. The latter were essentially of Alphatian culture, and must therefore have used a King title. However, the title of King is not used in Thyatis, except for foreign independent rulers, after the foundation of the Empire. Thus, the Alphatian "Kings" are actually translated as "Dukes", as it happens e.g. to Lornce M'Jozee in 1010 AC. The same is true for Norse Jarls, who are translated as Dukes (see Duke Stephan of Rhoona in Vestland, e.g.). Something similar happens with the Vatski dominions of Vyolstagrad and Stamtral, which are considered "Duchies".
My conclusion is that Thyatians typically translate any non-independent (or semi-independent) top level foreign noble title as "Duke".
Prince is used either as the heir of a ruler, or as a high-ranking ruler (but usually not a chief of state). This is often a translation of a local title -- for example, a "Prince" of Rockhome is actually known as a "Waiting Chief" or "Patient Chief" in the literal translation of the Dwarven title. Prince is a ruler title in Glantri, where it is used to replace King as the apical title, since there are multiple Princes (Glantri being an oligarchic republic, in essence), and in Nimmur (where it is obviously a translation of a local title, e.g., the Akkadian titles rubu or malku).
King is generally used for apical monarchs, except those who claim the title of Emperor. Emperor itself is obviously of Alphatian origin, although the word itself may be considered the Thyatian translation of the Alphatian title. IMO, what happened is that the original King of Thyatis was replaced by an Alphatian King. Thus, the Thyatians equated the two titles and translated the Alphatian King as "Rex" (I'll use the Latin word for king here to disambiguate). In modern times, however, the title is not used in Thyatis, and Duke and Archduke are the corresponding title to the Alphatian King, as well as to other non-sovereign Kings, such as the Rajahs and Maharajahs of Sind.
Emperor is also used as the translation of several similar styles, such as the Rajahdhirajah of Sind and other "King of Kings" styles.
References:
See the list of Mystaran nobles and their titles by year here.