I'm not sure : Provence is quite similar on this regard, and while it happened differently in Northern France, the privatisation of counties and duchies seems to have happened in roughly the same time at the exception of titles directly tied to royal authority such as Dux Francorum. I'd be interested on what
@Carp have to say about privatisation of counties and duchies or marches in Italy
Eh... early medieval Italian titles are not very consistent. In general, the Frankish conquest led to a shift from the Lombard
dux to the Carolingian
marchio among the provincial rulers of (northern) Italy, but there are so many exceptions to this rule that it hardly counts as a rule. Generally speaking,
dux existed only where it was inherited from pre-Carolingian times; the new feudal creations in Lombardy in the late 9th/10th centuries were all marches (Ivrea, Turin, Milan, Western and Eastern Liguria, etc.). Friuli, previously a Lombard duchy, became a march ruled by a
marchio, although Eberhard Unruoching was nevertheless described as "Duke of the March of Friuli." Tuscany likewise went from duchy to march, but the margraves of Tuscany were especially prone to compounding titles; the Bonifacii, who had begun as mere counts of Lucca, came to title themselves as "Count and Duke of Lucca and Margrave of Tuscany" or thereabouts.
Marchio alone was favored thereafter, but the temporary union of Tuscany (a march) and Spoleto (a duchy) under Humbert Bosonid and (after an interruption) his son Hugh was probably the reason for the adoption of
dux et marchio, a joint title which was apparently not abandoned even after Tuscany and Spoleto went their separate ways, such that Hugh's successors and the Canossa rulers of Tuscany in the 11th century still used
dux et marchio (although apparently Matilda of Canossa preferred the simpler title of Countess of Tuscany). Spoleto held on to its Lombard-era distinction as a duchy, probably because it was on the Carolingian periphery and part of the southern Lombard sphere where
dux and
princeps were preferred and
marchio was rare. Like Tuscany, however,
dux et marchio became preferred because of the conglomeration of the rather insignificant March of Camerino with the Duchy of Spoleto. Complicating matters is the fact that the
use of titles was not even consistent for individual people, who might be called
comes in one source and given a totally different style in another.
In other words, these titles had no fixed meaning or hierarchy (or at least none that can be discerned), save perhaps for a generally observed superiority of
dux, marchio, and
comes palatii over a "mere"
comes (although
comes was nevertheless sometimes paired with these other titles, particularly that of
marchio). Rather, titles tended to be used based on tradition and the personal ambitions and whims of their bearers. One sometimes gets the impression that
dux had an implication of autonomy not shared by
marchio, perhaps because of the pre-Carolingian provenance of
dux, but there is little reason to believe that
marchio was viewed as a strictly inferior title, particularly given the frequency with which the two were paired. I have read arguments before that
comes,
dux, and
marchio were ill-defined and overlapping terms that nevertheless had slightly different implications and associations, and thus Italian lords tended to accumulate and use them together to emphasize the totality of their power, but if so the nuances attached to these titles are hard for us to know. Nor, with the possible exception of
comes palatii (a relatively uncommon honor which sometimes - but not always - corresponds with proximity to or favor from the king/emperor), were they differentiated in terms of their ties to royal authority. The fact that the relatively obscure 10th century lords of Ravenna titled themselves
dux, for instance, does not appear to have distinguished them much from their neighbors in terms of their status, obligations, or links to royal authority. It seems likely to me that the style was merely a holdover from the Byzantine presence at Ravenna, much like the nearby Duke of Venice, although the weakness of royal control in Romagna in general may have also had something to do with it.