House Jerbiton members are said to be slightly better at expressing a modified version of their Casting Sigils. Basically a +3 to their Finesse roll. All because of the personal specialisation of Jerbiton the Founder.
I would call this a House quirk, a minor bonus to an aspect of Hermetic Magic that is either rarely used or has no significant game mechanical effects, and hence way too small to count as a Virtue.
Do any other Houses have similar quirks?
As an example how about House Guernicus:
Because they have such regimented training to pass their Apprentice Gauntlet (the same exam over centuries), that perhaps they have a slight bonus to casting ritual unique to House Guernicus.
Fenicil, who went through the same Apprentice Gauntlett, brought back several Cult of Mercury greater Rituals, and adapted them to be cast with Hermetic magic, as he understood it (Not able to integrate the rituals as well as Notatus did with the Aegis of the Hearth). So perhaps every House Guernicus member gets, say, a +3 bonus to cast Fenicil's rituals, the bonus is lost if a non-Guernicii participates (they are not quite in synch).
In practice, (only) elder Guernicus magi have access to Fenicil's rituals.
Mercere magi know how to build Mercere portals.
Tremere magi can call upon the resources of the House as per the rules in HoH:RL.
Etc.
Originally, the only house quirks not paid for with a virtue were tradeoffs: Bonisagi could take apprentices but incur an obligation to share research, Tremere got support from their House but had to spend seasons serving their House, Mercere magi had some weird social restrictions and benefits and Guernicus magi got to rule the Order but had various obligations and restrictions and still had to pay a Virtue for the privilege.
Over the course of publication, house quirks have become more common.
Thus, there are special spells that are Guernicus-only, just because. There are various Mastery Abilities that are restricted by House or faction, without any virtue or flaw being involved. Some virtues are restricted to a single House even when that House is not a Mystery Cult.
However, that is not what lvgreen said at all. Rather, that is +3 to the ease factor, equivalent to -3 to Finesse. It's also written about "artistic magi," not about House Jerbiton. Stated differently, artistic magi may alter their sigil slightly (a flourish) by taking a 3-point penalty to their Finesse attempt. I also don't see where this comes from the Founder; I do see that the Founder was skilled at altering perception in the next subsection, though.
Which Mastery Abilities are restricted to a single House? The Mutantum ones nearly are, but that's because the Virtue(s?) that give immediate access to them generally (not necessarily, though) only show up in House Mercere. And even if that Virtue(s?) hasn't shown up outside House Mercere any Bonisagus could theoretically change that pretty easily - might not want to, but could. But you said "without any Virtue," so you were referencing something else. Faction, yes, and requiring a Virtue, yes. But just by House?
Quaesitors get access to the Acute Sense mastery. The Quaesitors have a lot of overlap with House Guernicus, but they're not quite the same group.
Similarly, members of the Cult of Mercury get access to extra mastery abilities, which as a group overlaps with but isn't the same as magi with the Mercurian Magic virtue.
Sorry, I didn't have access to the books when I wrote from imperfect recollection.
Flourishes were under the Jerbiton chapter so I interpreted it as any Magus trained in the style of the Founder Jerbiton.
And I have difficulty thinking Ease Factors can be different - instead I visualise it as Magi trained in a particular way (which may only be fully appreciated with an artistic temperment), get an effective +3 to Finesse rolls to modifying their Flourish. Mathematically it comes out the same.
But the canon rule is the opposite. If you've learned to do it, you can modify your sigil by taking a -3 to Finesse rolls when doing the flourish. It's not an effective +3; it's an effective -3.
I don't have the books in front of me, but if I recalll correctly normal Magi have to do a Finesse roll against an Ease Factor of 9 for a Flourish, while a Jerbiton trained Magus does a Finesse roll against an Ease Factor of 6.
ie
Finesse roll against Ease Factor 9
vs
Finesse roll against modified Ease Factor 6
Since it is all Hermetic Magic, I prefer to think of it as all being the same Ease Factor, and Jerbiton training provides a modifier to the Finesse roll.
Finesse roll against modified Ease Factor 6 == Finesse roll modifier+3 against Ease Factor 6+3 (=9)
I must confess that the difficulty of getting into a medieval mindset, along with various edition rules has caused me to use assorted mental gymnastics when thinking about Ars Magica. It is probably not error-proof.
The undefined category "Artistic magi" don't get a bonus to flourishes, they get the possibility of doing flourishes.
The section (HoH:S p61) is part of the "Creation of Beautiful Things" section, which requires Finesse rolls whether you use Creo or Rego. Adding a flourish adds 3 to the Ease factor of this roll (which makes it harder).
No. The East Factor gets a +3. That means the target value is 3 higher than before. That means it's harder to achieve it with your Finesse roll. That is exactly the equivalent of getting a -3 on your Finesse.
I quoted it above. HoH:S Jerbiton section lists it as more difficult, not easier. I doubt it's more difficult for Jerbiton magi than for other magi. Presumably it's more difficult for everyone.
Other than changing "exactly" with "almost exactly", I agree with callen on this point. I think the spirit of what was written was the following. A sigil is not immutable. Any caster who cares can arbitrarily change the sigil within its "theme", at the cost of increasing by 3 the Ease Factor of the Int+Finesse roll involved. The effect is merely cosmetic. Crucially, what makes the caster recognizable is the sigil's theme, so in general a caster cannot exploit this flexibility to hide his identity.